#include <bits/stdc++.h>

using namespace std;

ifstream fin("harta.in");
ofstream fout("harta.out");

struct FlowEdge {
    int v, u;
    long long cap, flow = 0;
    FlowEdge(int v, int u, long long cap) : v(v), u(u), cap(cap) {}
};

class Graph {
private:
    const long long flow_inf = 1e18;
    vector<FlowEdge> edges;
    vector<vector<int>> adj;
    int n, m = 0;
    int s, t;
    vector<int> level, ptr;
    queue<int> q;

public:
    Graph(int n_, int s_, int t_): n(n_), s(s_), t(t_) {
        adj.resize(n);
        level.resize(n);
        ptr.resize(n);
    }

    void add_edge(int v, int u, long long cap) {
        edges.emplace_back(v, u, cap);
        edges.emplace_back(u, v, 0);

        adj[v].push_back(m);
        adj[u].push_back(m + 1);
        m += 2;
    }

    bool bfs() {
        while (!q.empty()) {
            int v = q.front();
            q.pop();
            for (int id : adj[v]) {
                if (edges[id].cap - edges[id].flow < 1)
                    continue;
                if (level[edges[id].u] != -1)
                    continue;
                level[edges[id].u] = level[v] + 1;
                q.push(edges[id].u);
            }
        }
        return level[t] != -1;
    }

    long long dfs(int v, long long pushed) {
        if (pushed == 0)
            return 0;
        if (v == t)
            return pushed;
        for (int& cid = ptr[v]; cid < (int)adj[v].size(); cid++) {
            int id = adj[v][cid];
            int u = edges[id].u;
            if (level[v] + 1 != level[u] || edges[id].cap - edges[id].flow < 1)
                continue;
            long long tr = dfs(u, min(pushed, edges[id].cap - edges[id].flow));
            if (tr == 0)
                continue;
            edges[id].flow += tr;
            edges[id ^ 1].flow -= tr;
            return tr;
        }
        return 0;
    }

    long long flow() {
        long long f = 0;
        while (true) {
            fill(level.begin(), level.end(), -1);
            level[s] = 0;
            q.push(s);
            if (!bfs())
                break;
            fill(ptr.begin(), ptr.end(), 0);
            while (long long pushed = dfs(s, flow_inf)) {
                f += pushed;
            }
        }
        return f;
    }

    vector<pair<int,int>> matching() {
        vector<pair<int,int>> match;
        for(auto& it: edges) {
            if(it.flow == 1 && it.flow == it.cap && it.v != s && it.u != t && it.v != it.u) {
                match.push_back(make_pair(it.v, it.u));
            }
        }
        return match;
    }
};

int main() {
    int n;
    fin >> n;
    Graph dinic(n * 2 + 2, 0, n + n + 1);

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

    for(int x, y, i = 1; i <= n; i++) {
        fin >> x >> y;
        dinic.add_edge(0, i, x);
        dinic.add_edge(i + n, n + n + 1, y);
    }

    fout << dinic.flow() << '\n';

    auto rasp = dinic.matching();
    for(auto &muchie: dinic.matching()) {
        fout << muchie.first << ' ' << muchie.second - n << '\n';
    }
    return 0;
}