#include <bits/stdc++.h>
using namespace std;

ifstream f("harta.in");
ofstream g("harta.out");
#define cin f
#define cout g

int n, m;
vector<int> prevs(202);
vector<vector<int>> capacity(202, vector<int>(202));
vector<vector<int>> adj(202);

int bfs(int s, int t, vector<int>& prevs) {
    fill(prevs.begin(), prevs.end(), -1);
    prevs[s] = -2;
    queue<pair<int, int>> q;
    q.push({s, INT_MAX});

    while (!q.empty()) {
        int curr = q.front().first;
        int flow = q.front().second;
        q.pop();

        for (int next : adj[curr]) {
            if (prevs[next] == -1 && capacity[curr][next]) {
                prevs[next] = curr;
                int new_flow = min(flow, capacity[curr][next]);
                if (next == t)
                    return new_flow;
                q.push({next, new_flow});
            }
        }
    }
    return 0;
}

int maxflow(int s, int t) {
    int flow = 0;
    int new_flow;

    while (new_flow = bfs(s, t, prevs)) {
        flow += new_flow;
        int curr = t;
        while (curr != s) {
            int prev = prevs[curr];
            capacity[prev][curr] -= new_flow;
            capacity[curr][prev] += new_flow;
            curr = prev;
        }
    }

    return flow;
}

int main() {
	cin >> n;
    int sink = 2 * n + 1;
	for(int i = 1; i <= n; i ++) {
		int a, b;
		cin >> a >> b;

        adj[0].push_back(i);
        adj[i].push_back(0);
        capacity[0][i] = a;

        adj[n+i].push_back(sink);
        adj[sink].push_back(n+i);
        capacity[n+i][sink] = b;
	}

    for (int i = 1; i<= n; i++){
        for (int j = n + 1; j < sink; j++){
            if (j - i == n) continue;
            adj[j].push_back(i);
            adj[i].push_back(j);
            capacity[i][j] = 1;
        }
    }

	cout << maxflow(0, sink) << '\n';
    for (int i = 1; i <= n; i++){
        for (int j = n + 1; j < sink; j++){
            if (capacity[j][i] == 1)
                cout << i << ' ' << j - n << '\n';
        }
    }
	return 0;
}