#include <fstream>
#include <string.h>
using namespace std;
int n,m,i,j,k,od,a[101],b[101],q[101],l[101][101];
int main()
{ifstream in("harta.in");
ofstream w("harta.out");
in>>n;
for(i=1;i<=n;++i)
  {in>>a[i]>>b[i];
  m+=a[i];}
w<<m<<"\n";
for(i=1;i<=n;++i)
  {memset(q,0,sizeof(q));
  for(j=1;j<=n;++j)
    if(b[j]&&i!=j)
     {l[b[j]][++q[b[j]]]=j;}
  for(j=n;j&&a[i];--j)
    for(k=q[j];k&&a[i];--k)
      {od=l[j][k];
      a[i]--;
      b[od]--;
      w<<i<<" "<<od<<"\n";}}
return 0;}