#include <fstream>
#include <random>
using namespace std;
ifstream cin("harta.in");
ofstream cout("harta.out");
int in[105],out[105];
bool ok[105][105];
int main()
{
    int n,m=0;
    cin>>n;
    for(int i=1;i<=n;++i)
    {
        cin>>out[i]>>in[i];
        m+=out[i]+in[i];
    }
    m/=2;
    cout<<m<<'\n';
    for(int i=1;i<=m;++i)
    {
        int x=rand()%n+1;
        int y=rand()%n+1;
        while(y==x) y=rand()%n+1;
        while(ok[x][y]==1 or x==y) x=rand()%n+1;
        ok[x][y]=1;
        cout<<x<<' '<<y<<'\n';
    }
    return 0;
}