#include <bits/stdc++.h>
#define N 1004
#define inf 2e9

using namespace std;

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

int n, m;
int sursa;
struct muchie{
    int f, c;
} M[N][N];
vector<int> g[N], h[N];

void Citire()
{
    int i, j;
    int x, y;
    fin >> m;
    sursa = 0;
    n = 2 * m + 2;
    for( i=1; i<=m; i++ ){
        fin >> x >> y;
        g[sursa].push_back(i);  M[sursa][i] = {0, x};
        h[i].push_back(sursa);  M[i][sursa] = {0, x};

        g[i + m].push_back(n); M[i + m][n] = {0, y};
        h[n].push_back(i + m); M[n][i + m] = {0, y};
    }

    for( i=1; i<=m; i++ )
        for ( j=1; j<=m; j++ ) if (i != j) {
            g[i].push_back(j + m); M[i][j + m] = {0, 1};
            h[j + m].push_back(i); M[j + m][i] = {0, 1};
        }
}

queue<int> Q;
int Dad[N];
bool viz[N];
bool BFS()
{
    int i;
    for( i=0; i<=n; i++ )
        Dad[i] = inf, viz[i] = 0;
    while(!Q.empty()) Q.pop();
    Q.push(sursa);
    viz[sursa] = 1;
    while( !Q.empty() )
    {
        int nod = Q.front();
        Q.pop();
        for( auto next : g[nod] ) /// arc direct
            if( viz[next] == 0 && M[nod][next].c > M[nod][next].f ){
                Q.push(next);
                viz[next] = 1;
                Dad[next] = nod;
                if( next == n ) return true;
            }
        for( auto next : h[nod] ) /// arc invers
            if( viz[next] == 0 && M[nod][next].f > 0 ){
                Q.push(next);
                viz[next] = 1;
                Dad[next] = -nod;
                if( next == n ) return true;
            }
    }
    return 0;
}

void Revizuire( int x, int &mn )
{
    if( x == sursa ) return;
    if( Dad[x] >= 0 ){
        mn = min(mn, M[Dad[x]][x].c - M[Dad[x]][x].f);
        Revizuire( Dad[x], mn );
        M[Dad[x]][x].f += mn;
        M[x][Dad[x]].f += mn;
    }
    if( Dad[x] < 0 ){
        mn = min(mn, M[-Dad[x]][x].f);
        Revizuire( -Dad[x], mn );
        M[-Dad[x]][x].f -= mn;
        M[x][-Dad[x]].f -= mn;
    }
}

void Flux()
{
    while( BFS() ){
        int mn = inf;
        Revizuire(n, mn);
    }
}

void Rezolvare() {
    Flux();
    int flux = 0;
    for( auto nod : g[sursa] ) flux += M[sursa][nod].f;
    fout << flux << "\n";


    for (int i = 1; i <= m; i++)
        for (int j = 1; j <= m; j++) {
            if (M[i][j + m].f) fout << i << " " << j << "\n";
        }
}

int main()
{
    Citire();
    Rezolvare();
    return 0;
}