#include<fstream.h>
int a[100],b[100],n,c[100][100],nr1;
void citire(){
ifstream fin("harta.in");
fin>>n;
for (int i=0;i<n;i++){
   fin>>a[i]>>b[i];
   nr1+=a[i];}
fin.close();
}
void caut(){
ofstream fout("harta.out");
fout<<nr1<<"\n";
  for (int i=0;i<n;i++){
      int nr=a[i];
      int j=0;
      while (nr>0&&j<i){
	if (b[j]!=0&&c[i][j]==0){
	       b[j]--;
	       nr--;
	       c[i][j]=1;
	       fout<<i+1<<" "<<j+1<<"\n";}
	   j++;}
      j=i+1;
     while (nr>0&&j<n){
	   if (b[j]!=0&&c[i][j]==0){
	       b[j]--;
	       nr--;
	       c[i][j]=1;
	       fout<<i+1<<" "<<j+1<<"\n";}
	   j++;}
}
fout.close();
}
int main(){
citire();
caut();
return 0;
}