#include <cstdio>
#include <vector>
#define nmax 105
using namespace std;

vector <int> g[nmax];
vector <int> r[nmax];
int input[nmax],output[nmax];
int n,sum;

int main() {
    freopen("harta.in","r",stdin);
    freopen("harta.out","w",stdout);
    scanf("%d",&n);
    for (int i=1;i<=n;i++) {
        scanf("%d %d",&output[i],&input[i]);
        sum += input[i];
    }
    for (int i=1;i<=n;i++) for (int j=1;j<=n;j++) if (i != j && output[i] > 0 && input[j] > 0) {
        g[i].push_back(j);
    }
    bool changed = true;
    while (changed) {
        changed = false;
        for (int i=1;i<=n;i++) {
            if (output[i] > 0) {
                int x = g[i].back(); g[i].pop_back();
                if (input[x] <= 0) continue;
                r[i].push_back(x);
                input[x]--;
                output[i]--;
                changed = true;
            }
        }
    }
    printf("%d\n",sum);
    for (int i=1;i<=n;i++) {
        for (vector <int> :: iterator it = r[i].begin();it != r[i].end();it++) {
            printf("%d %d\n",i,*it);
        }
    }
    return 0;
}