#include <fstream>
#include <vector>
#include <queue>

using namespace std;
ifstream f("harta.in");
ofstream g("harta.out");
int n,X,Y,m;
int st,dr;
const int N=205;
int viz[N],last[N];
int d[N][N],flux[N][N];
vector<int>a[N];
bool bfs()
{
    for(int i=0;i<=2*n+1;i++)
        viz[i]=0,last[i]=-1;
    queue<int>q;
    q.push(st);
    while(!q.empty())
    {
        int nod=q.front();
        q.pop();
        if(nod==dr)
            continue;
        for(auto x: a[nod])
        {
          //  g<<nod<<" "<<x<<'\n';
            if(!viz[x]&&d[nod][x]>flux[nod][x])
            {
                viz[x]=true;
                last[x]=nod;
                q.push(x);
            }
        }
    }
    return viz[dr];
}
int main()
{
    f>>n;
    st=0;
    dr=2*n+1;
    int sum=0;
    for(int i=1;i<=n;i++)
    {
        f>>X>>Y;
        sum+=X;
        a[st].push_back(i);
        a[i].push_back(st);
        a[dr].push_back(i+n);
        a[i+n].push_back(dr);
        d[st][i]=X;
        d[i+n][dr]=Y;
    }
    g<<sum<<'\n';
    for(int i=1;i<=n;i++)
        for(int j=1;j<=n;j++)
        if(i!=j)
        {
            a[i].push_back(j+n);
            a[j+n].push_back(i);
            d[i][j+n]=1;
        }
    while(bfs())
    {
        for(int i=dr;i!=st;i=last[i])
        {
            flux[last[i]][i]++;
            flux[i][last[i]]--;
        }
    }
    for(int i=1;i<=n;i++)
        for(int j=n+1;j<=2*n;j++)
            if(flux[i][j]!=0)
                g<<i<<" "<<j-n<<'\n';
    return 0;
}