#include <fstream>
using namespace std;
ifstream in("harta.in");
ofstream out("harta.out");
int grad_in[105],grad_out[105],n,i,j,x,y,a[101][101];
void citire()
{in>>n;
for(i=1;i<=n;i++)
 in>>grad_out[i]>>grad_in[i];}

int main()
{int contor=0,k;
citire();
  for(i=1;i<=n;i++)
  {k=1;
  while(grad_out[i]!=0 && k<=n)
    {if(k!=i && a[k][i]!=1 && grad_in[k]>0)  {a[i][k]=1;grad_in[k]--;grad_out[i]--;
                                               contor++;
                                             } k++;  }

    }
               out<<contor<<'\n';
               for(i=1;i<=n;i++)
                for(j=1;j<=n;j++)
                 if(a[i][j]==1) out<<i<<' '<<j<<'\n';

      return 0;
}