#include <fstream>
#include <cstring>
using namespace std;
ifstream cin("harta.in");
ofstream cout("harta.out");

struct celula{

int nod;
celula *next;
}*g[1001],*a;

int n,m,x,y,cost,i,flow,j;
int c[1001][1001];
int t[1001],q[1001];
bool viz[1001];

void update()
{
    int mini=2000000000;
    int aux=2*n+1;

    while(aux>1)
    {
        mini=min(mini,c[t[aux]][aux]);
        aux=t[aux];
    }

    flow+=mini;
    aux=2*n+1;

    while(aux>1)
    {
        c[t[aux]][aux]-=mini;
        c[aux][t[aux]]+=mini;
        aux=t[aux];
    }

}
bool bfs()
{
    q[1]=0;
    int l=1, r=1;
    int nod=1;
    int ok=0;

    memset(viz,0,sizeof(viz));
    viz[0]=1;

    while(l<=r)
    {
        nod=q[l];
        l++;

        for(celula *p=g[nod]; p!=NULL; p=p->next)
        {
            if(viz[p->nod]==0 && c[nod][p->nod]>0)
            {
                t[p->nod]=nod;
                if(p->nod==2*n+1) update(), ok=1;
                else
                q[++r]=p->nod, viz[p->nod]=1;
            }
        }
    }

    return ok;

}

int main()
{
    cin>>n;
    for(i=1;i<=n;++i)
    {
        cin>>x>>y;

        a = new celula;
        a->nod=i;
        a->next=g[0];
        g[0]=a;

        c[0][i]=x;

        a= new celula;
        a->nod=0;
        a->next=g[i];
        g[i]=a;

        c[i][0]=0;

        a= new celula;
        a->nod=2*n+1;
        a->next=g[n+i];
        g[n+i]=a;
        c[n+i][2*n+1]=y;

        a=new celula;
        a->nod=n+i;
        a->next=g[2*n+1];
        g[2*n+1]=a;
        c[2*n+1][n+i]=0;

    }

    for(i=1;i<=n;++i)
    {
        for(j=n+1;j<=2*n;++j)
        {
            if(j==n+i)
            continue;
            else
            {
            a=new celula;
            a->nod=i;
            a->next=g[j];
            g[j]=a;
            c[i][j]=1;

            a= new celula;
            a->nod=j;
            a->next=g[i];
            g[i]=a;
            c[j][i]=0;
            }

        }
    }


    while(bfs())
    {

    }

    cout<<flow<<"\n";

    for(i=1;i<=n;++i)
        for(j=n+1;j<=2*n;++j)
    {
        if(j!=n+i)
        if(c[i][j]==0) cout<<i<<" " << j-n<<"\n";
    }
    return 0;
}