#include <cstdio>
#include <vector>
#include <algorithm>

using namespace std;

#define file_in "harta.in"
#define file_out "harta.out"

#define nmax (1<<8)

int N,s,d,viz[nmax],C[nmax][nmax],F[nmax][nmax],coada[nmax],vmin,i,j;


int bfs(){

	int p=0,u=1,i,g=0;
	for (i=1;i<=d;++i) viz[i]=-1;

	coada[1]=0;
	viz[s]=0;
	
	while(p<=u){
		for(i=0;i<=d;++i){
			if( C[coada[p]][i]>F[coada[p]][i] && viz[i]==-1){
				if(i==d){
					g=1;
					continue;
				}
				coada[++u]=i;
				viz[i]=coada[p];
			}
		}
		p++;
	}
	
	return g;
}

				 

int main(){
	
	int x,y,suma;
	
	freopen(file_in,"r",stdin);
	freopen(file_out,"w",stdout);
	
	scanf("%d", &N);
	s=0;
	suma=0;
	d=2*N+1;
	for (i=1;i<=N;++i){
		 scanf("%d %d", &x, &y);
		 suma+=x;
		 C[s][i]=x;
		 C[i+N][d]=y;
		 for (j=N+1;j<d;++j) 
			  if (i!=j-N)
				   C[i][j]=1;
	}
	
	while(bfs()){
		for (i=N+1;i<d;++i)
			 if (viz[i]){
				 viz[d]=i;
				 vmin=0x3f3f3f3f;
				 for (j=d;j!=0;j=viz[j])
					  vmin=min(vmin,C[viz[j]][j]-F[viz[j]][j]);
				 for (j=d;j!=0;j=viz[j])
					  F[viz[j]][j]+=vmin,
					  F[j][viz[j]]-=vmin;
			 }
	}
	
	printf("%d\n", suma);
	for (i=1;i<=N;++i)
		 for (j=N+1;j<d;++j)
			  if (i!=j-N && F[i][j]>0)
				   printf("%d %d\n", i, j-N);
	
	return 0;

}