#include<fstream>
#include<algorithm>
#include<vector>
#include<queue>
using namespace std;
ifstream f("harta.in");
ofstream g("harta.out");
vector<int>a[205];
int n,s,d,sol,r[205][205],tata[205],in[205],out[205];
bool viz[205];
queue<int>q;

inline bool bfs()
{
    int i,x,l;
    for(i=1;i<=n;i++)
        viz[i]=0,tata[i]=0;
    q.push(s);
    viz[s]=1;
    while(!q.empty())
    {
        x=q.front();
        q.pop();
        l=a[x].size();
        for(i=0;i<l;i++)
        if(viz[a[x][i]]==0&&r[x][a[x][i]]>0)
        {
            viz[a[x][i]]=1;
            tata[a[x][i]]=x;
            q.push(a[x][i]);
        }
    }
    return viz[d];
}

inline void flux_maxim()
{
    int i,j,flow,l=a[d].size();
    while(bfs()!=0)
    {
        for(i=0;i<l;i++)
        if(tata[a[d][i]]!=0&&r[a[d][i]][d]>0)
        {
            flow=r[a[d][i]][d];
            for(j=a[d][i];j!=s;j=tata[j])
            {
                flow=min(flow,r[tata[j]][j]);
                if(flow==0)
                    break;
            }
            if(flow!=0)
            {
                r[a[d][i]][d]-=flow;
                r[d][a[d][i]]+=flow;
                for(j=a[d][i];j!=s;j=tata[j])
                {
                    r[tata[j]][j]-=flow;
                    r[j][tata[j]]+=flow;
                }
                sol+=flow;
            }
        }
    }
}

int main()
{
    int i,sum=0,j,l,nn;
    f>>n;
    nn=n;
    for(i=1;i<=n;i++)
    {
        f>>out[i]>>in[i];
        sum+=in[i];
    }
    s=2*n+1;
    d=2*n+2;
    for(i=1;i<=n;i++)
    {
        a[s].push_back(i);
        a[i].push_back(s);
        a[i+n].push_back(d);
        a[d].push_back(i+n);
        r[s][i]=out[i];
        r[i+n][d]=in[i];
        for(j=1;j<=n;j++)
        if(i!=j)
        {
            a[i].push_back(j+n);
            a[j+n].push_back(i);
            r[i][j+n]=1;
        }
    }
    n=n*2+2;
    flux_maxim();;
    g<<sum<<'\n';
    for(i=1;i<=nn;i++)
    {
        l=a[i].size();
        for(j=0;j<l;j++)
        if(a[i][j]<=2*nn&&r[i][a[i][j]]==0)
            g<<i<<" "<<a[i][j]-nn<<'\n';
    }
    return 0;
}