#include <iostream>
#include <fstream>
#include <algorithm>
#include <vector>
#include <queue>

///#include <tryhardmode>
///#include <GODMODE::ON>

using namespace std;

ifstream fin ("harta.in");
ofstream fout ("harta.out");

const int INF=2e9;
const int NMAX=4e2+5;

vector<int>v[2*NMAX];
bool viz[2*NMAX];
int t[NMAX];
int flux[NMAX][NMAX];

int n,s,d;

bool bfs(int p)
{
    int i;
    for(i=0;i<=2*n+2;i++)
        viz[i]=false;
    queue<int>q;
    q.push(p);
    viz[p]=true;
    while(!q.empty())
    {
        int p=q.front();
        q.pop();
        if(p==d)
            return true;
        for(auto i:v[p])
        {
            if(!viz[i] && flux[p][i]>0)
            {
                q.push(i);
                t[i]=p;
                viz[i]=true;
            }
        }
    }
    return false;
}

int main()
{
    int m,i,j,mini=INF,x,y;
    long long maxi=0;
    fin>>n;
    s=0;
    d=2*n+1;
    for(i=1;i<=n;i++)
    {
        fin>>x>>y;
        v[s].push_back(i);
        v[i].push_back(s);
        v[i+n].push_back(d);
        v[d].push_back(i+n);
        flux[s][i]=x;
        flux[i+n][d]=y;
        for(j=1;j<=n;j++)
            if(i==j)
                continue;
            else
            {
                flux[i][j+n]=1;
                v[i].push_back(j+n);
                v[j+n].push_back(i);
            }
    }
    while(bfs(s))
    {
        for(auto i:v[d])
        {
            if(!viz[i] || flux[i][d]<=0)
                continue;
            mini=INF;
            t[d]=i;
            x=d;
            while(x!=s)
            {
                mini=min(mini,flux[t[x]][x]);
                x=t[x];
            }
            if(mini==0)
                continue;
            maxi+=mini;
            x=d;
            while(x!=s)
            {
                flux[x][t[x]]+=mini;
                flux[t[x]][x]-=mini;
                x=t[x];
            }
        }
    }
    fout<<maxi<<"\n";
    for(i=1;i<=n;i++)
    {
        for(j=1;j<=n;j++)
            if(flux[j+n][i]==1)
                fout<<i<<" "<<j<<"\n";
    }
    return 0;
}