#include <fstream>
using namespace std;

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

int main() {
    int N, S = 0, in[105], out[105];
    int mat[105][105], freq[105];

    fin >> N;

    for(int i = 1; i <= N; i++) {
        fin >> in[i] >> out[i];
        S += in[i];
    }

    fout << S << '\n';

    for(int i = 1; i <= N; i++) {
        fill(freq + 0, freq + N + 1, 0);
        for(int j = 1; j <= N; j++) {
            if(out[j] && i != j) {
                mat[out[j]][++freq[out[j]]] = j;
            }
        }

        for(int j = 1; j <= N; j++) {
            for(int it = freq[j]; it != 0 && in[i] != 0; it--) {
                int nod = mat[j][it];
                in[i]--;
                out[nod]--;
                fout << i << " " << nod << '\n';
            }
        }
    }
    return 0;
}