#include <cstdio>
#include <cstring>
#define INF 1000000000
#define MAXN 100
bool viz[2*MAXN+2];
int from[2*MAXN+2], s, d, n, q[2*MAXN+2], c[2*MAXN+2][2*MAXN+2], f[2*MAXN+2][2*MAXN+2];
inline bool bfs(){
    int st, dr, x;
    st=0;
    q[0]=s;
    dr=1;
    memset(viz, 0, sizeof viz);
    memset(from, 0, sizeof from);
    viz[s]=1;
    while((st<dr)&&(viz[d]==0)){
        for(x=1; x<=2*n+1; x++){
            if((viz[x]==0)&&(c[q[st]][x]>f[q[st]][x])){
                viz[x]=1;
                from[x]=q[st];
                q[dr++]=x;
            }
        }
        st++;
    }
    return viz[d];
}
int main(){
    int i, j, min, x, ans;
    FILE *fin, *fout;
    fin=fopen("harta.in", "r");
    fout=fopen("harta.out", "w");
    fscanf(fin, "%d", &n);
    s=0;
    d=2*n+1;
    for(i=1; i<=n; i++){
        for(j=1; j<=n; j++){
            if(i!=j){
                c[i][j+n]=1;
            }
        }
        fscanf(fin, "%d%d", &c[s][i], &c[i+n][d]);
    }
    ans=0;
    while(bfs()){
        for(x=0; x<=2*n; x++){
            if((viz[x])&&(c[x][d]>f[x][d])){
                from[d]=x;
                min=INF;
                i=d;
                while(i!=s){
                    if(min>c[from[i]][i]-f[from[i]][i]){
                        min=c[from[i]][i]-f[from[i]][i];
                    }
                    i=from[i];
                }
                ans+=min;
                i=d;
                while(i!=s){
                    f[from[i]][i]+=min;
                    f[i][from[i]]-=min;
                    i=from[i];
                }
            }
        }
    }
    fprintf(fout, "%d\n", ans);
    for(i=1; i<=n; i++){
        for(j=n+1; j<=2*n; j++){
            if(f[i][j]==1){
                fprintf(fout, "%d %d\n", i, j-n);
            }
        }
    }
    fclose(fin);
    fclose(fout);
    return 0;
}