#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

#define ar array
#define vt vector
#define pq priority_queue
#define pu push
#define pub push_back
#define em emplace
#define emb emplace_back
#define mt make_tuple

#define all(x) x.begin(), x.end()
#define allr(x) x.rbegin(), x.rend()
#define allp(x, l, r) x.begin() + l, x.begin() + r
#define len(x) (int)x.size()
#define uniq(x) unique(all(x)), x.end()

using ll = long long;
using ld = long double;
using ull = unsigned long long;

template<class Fun> class y_combinator_result {
    Fun fun_;
public:
    template<class T> explicit y_combinator_result(T &&fun): fun_(std::forward<T>(fun)) {}
    template<class ...Args> decltype(auto) operator()(Args &&...args) { return fun_(std::ref(*this), std::forward<Args>(args)...); }
};
template<class Fun> decltype(auto) y_combinator(Fun &&fun) { return y_combinator_result<std::decay_t<Fun>>(std::forward<Fun>(fun)); }

template <class T, size_t N>
void re(array <T, N>& x);
template <class T> 
void re(vt <T>& x);

template <class T> 
void re(T& x) {
    cin >> x;
}

template <class T, class... M> 
void re(T& x, M&... args) {
    re(x), re(args...);
}

template <class T> 
void re(vt <T>& x) {
    for(auto& it : x) re(it);
}

template <class T, size_t N>
void re(array <T, N>& x) {
    for(auto& it : x) re(it);
}

template <class T, size_t N>
void wr(const array <T, N>& x);
template <class T> 
void wr(const vt <T>& x);

template <class T> 
void wr(const T& x) {
    cout << x;
}

template <class T, class ...M>  
void wr(const T& x, const M&... args) {
    wr(x), wr(args...);
}

template <class T> 
void wr(const vt <T>& x) {
    for(auto it : x) wr(it, ' ');
}

template <class T, size_t N>
void wr(const array <T, N>& x) {
    for(auto it : x) wr(it, ' ');
}

template<class T, class... M>
auto mvt(size_t n, M&&... args) {
    if constexpr(sizeof...(args) == 1)
        return vector<T>(n, args...);
    else
        return vector(n, mvt<T>(args...));
}

void set_fixed(int p = 0) {
    cout << fixed << setprecision(p);
}

void set_scientific() {
    cout << scientific;
}

void Open(const string& name) {
#ifndef ONLINE_JUDGE
    (void)!freopen((name + ".in").c_str(), "r", stdin);
    (void)!freopen((name + ".out").c_str(), "w", stdout);
#endif
}

const ll INF = 1e18;

struct Dinic {
    int n, s, t, mxid;
    vt <int> d;
    vt <int> done;
    vt <vt <int>> g;
    vt  <vt <ll>> cap, flow;
    Dinic() {
    }
    Dinic(int _n) {
        n = _n + 10;
        mxid = 0;
        g.resize(n);
        cap.resize(n, vt <ll>(n));
        flow.resize(n, vt <ll>(n));
    }

    void add_edge(int u, int v, ll w) {
        g[u].emb(v);
        g[v].emb(u);
        cap[u][v] = w;
    }

    bool bfs() {
        d.assign(n, -1);
        d[s] = 0;
        queue <int> q;
        q.pu(s);
        while(!q.empty()) {
            int u = q.front();
            q.pop();
            for(auto &v : g[u]) {
                if(d[v] == -1 && cap[u][v] > flow[u][v]) 
                    d[v] = d[u] + 1, q.pu(v);
            }
        }
        return d[t] != -1;
    }

    ll dfs(int u, ll fflow) {
        if (u == t) {
            return fflow;
        }

        for(int &i = done[u]; i < len(g[u]);i++) {
            int v = g[u][i];

            if(cap[u][v] <= flow[u][v]) 
                continue;

            if(d[v] == d[u] + 1) {
                ll nw = dfs(v, min(fflow, cap[u][v] - flow[u][v]));
                if(nw > 0) {
                    flow[u][v] += nw;
                    flow[v][u] -= nw;
                    return nw;
                }
            }
        }
        return 0;
    }

    ll max_flow(int _s, int _t) {
        s = _s;
        t = _t;
        ll fflow = 0;
        while (bfs()) {
            done.assign(n, 0);
            while(ll nw = dfs(s, INF)) {
                fflow += nw;
            }
        }
        return fflow;
    }
};

void solve() {
    int n; re(n);
    Dinic G(2 * n + 10);
    int s = 2 * n + 1, t = 2 * n + 2;
    for (int u = 0; u < n; ++u) {
        int out, in; re(out, in);
        G.add_edge(u + n, t, in);
        G.add_edge(s, u, out);
    }

    for (int i = 0; i < n; ++i)
        for (int j = 0; j < n; ++j) {
            if (i != j) G.add_edge(i, j + n, 1);
        }

    wr(G.max_flow(s, t), '\n');
    for (int i = 0; i < n; ++i)
        for (int j = 0; j < n; ++j)
            if (i != j && G.cap[i][j + n] - G.flow[i][j + n] == 0)
                wr(i + 1, ' ', j + 1, '\n');
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    Open("harta");

    int t = 1;
    for(;t;t--) {
        solve();
    }
    
    return 0;
}