#include <fstream>
#include <queue>
#include <vector>
#define inf 2000000000
using namespace std;
ifstream fin("harta.in");
ofstream fout("harta.out");
vector<int> g[205];
queue<int> q;
int tata[205];
int f[205][205],c[205][205],cost[205][205];
struct grad
{
    int in, out;
}v[101];
bool viz[205];
int n,m,x,y;
long long sol;
bool bfs(int st, int dst)
{
    q.empty();
    q.push(st);
    for(int i=1;i<=2*n+1;i++)
        viz[i]=tata[i]=0;
    viz[st]=1;
    while(!q.empty())
    {
        int nod=q.front();
        q.pop();
        for(int i=0;i<g[nod].size();i++)
        {
            int nou=g[nod][i];
            if(viz[nou]==0&&c[nod][nou]-f[nod][nou]>0)
            {
                viz[nou]=1;
                tata[nou]=nod;
                q.push(nou);
            }
        }
    }
    return viz[dst];
}
int minim(int nod)
{
    int Min=c[nod][2*n+1]-f[nod][2*n-1];
    while(nod!=0)
    {
        int nou=tata[nod];
        Min=min(Min,c[nou][nod]-f[nou][nod]);
        nod=nou;
    }
    return Min;
}
void add(int val,int nod)
{
    f[nod][2*n+1]+=val;
    f[2*n+1][nod]-=val;
    while(nod!=0)
    {
        int nou=tata[nod];
        f[nou][nod]+=val;
        f[nod][nou]-=val;
        nod=nou;
    }
}
int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
    {
        fin>>v[i].in>>v[i].out;
        m+=v[i].in;
    }
    fout<<m<<'\n';
    for(int i=1;i<=n;i++)
    {
        g[0].push_back(i);
        g[i].push_back(0);
        g[n+i].push_back(2*n+1);
        g[2*n+1].push_back(n+i);
        c[0][i]=v[i].in;
        c[i+n][2*n+1]=v[i].out;
    }
    for(int i=1;i<=n;i++)
        for(int j=n+1;j<=2*n;j++)
            if(i!=j-n)
            {
                g[i].push_back(j);
                g[j].push_back(i);
                c[i][j]=1;
            }
    while(bfs(0,2*n+1))
        for(int i=0;i<g[2*n+1].size();i++)
        {
            int nod=g[2*n+1][i];
            if (f[nod][2*n+1]==c[nod][2*n+1]||viz[nod]==0)
                continue;
            int qmin=minim(nod);
            add(qmin,nod);
        }
    for(int i=1;i<=n;i++)
        for(int j=n+1;j<=2*n;j++)
            if(f[i][j]>0)
                fout<<i<<' '<<j-n<<'\n';
    return 0;
}