#import<fstream>
#import<string.h>
using namespace std;int n,m,i,j,k,o,a[101],b[101],q[101],l[101][101];main(){ifstream I("harta.in");ofstream w("harta.out");I>>n;for(i=1;i<=n;++i){I>>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)o=l[j][k],a[i]--,b[o]--,w<<i<<" "<<o<<"\n";}}