#include <fstream>
#include <vector>
#include <queue>
using namespace std;
struct muchie{
	int x, y;
};
int cap[205][205], flow[205][205], f[205];
vector <muchie> ras;
vector <int> v[205];
bool bfs( int start, int target ){
	int i, x, y;
	for( i = 0; i <= target; i++ ){
		f[i] = INT32_MAX / 3;
	}
	f[start] = 0;
	queue <int> q;
	q.push( start );
	while( q.empty() == false ){
		x = q.front();
		q.pop();
		for( i = 0; i < v[x].size(); i++ ){
			y = v[x][i];
			if( flow[x][y] < cap[x][y] && f[x] + 1 < f[y] ){
				f[y] = f[x] + 1;
				q.push( y );
			}
		}
	}
	return f[target] != INT32_MAX / 3;
}
int maxflow( int x, int target, int max_flow ){
	int i, y, tot_flow, curr_flow;
	if( max_flow == 0 ){
		return 0;
	}
	if( x == target ){
		return max_flow;
	}
	tot_flow = 0;
	for( i = 0; i < v[x].size(); i++ ){
		y = v[x][i];
		if( f[x] + 1 != f[y] ){
			continue;
		}
		curr_flow = maxflow( y, target, min( max_flow - tot_flow, cap[x][y] - flow[x][y] ) );
		flow[x][y] += curr_flow;
		flow[y][x] -= curr_flow;
		tot_flow += curr_flow;
	}
	return tot_flow;
}
int main(){
	int n, i, j;
	ifstream fin( "harta.in" );
	ofstream fout( "harta.out" );
	fin >> n;
	for( i = 1; i <= n; i++ ){
		fin >> cap[0][i] >> cap[i + n][2 * n + 1];
		v[0].push_back( i );
		v[i].push_back( 0 );
		v[i + n].push_back( 2 * n + 1 );
		v[2 * n + 1].push_back( i + n );
	}
	for( i = 1; i <= n; i++ ){
		for( j = 1; j <= n; j++ ){
			if( i != j ){
				v[i].push_back( j + n );
				v[j + n].push_back( i );
				cap[i][j + n] = 1;
			}
		}
	}
	while( bfs( 0, 2 * n + 1 ) ){
		maxflow( 0, 2 * n + 1, INT32_MAX / 3 );
	}
	for( i = 1; i <= n; i++ ){
		for( j = 1; j <= n; j++ ){
			if( i != j && flow[i][j + n] == cap[i][j + n] ){
				ras.push_back( { i, j } );
			}
		}
	}
	fout << ras.size() << '\n';
	for( i = 0; i < ras.size(); i++ ){
		fout << ras[i].x << ' ' << ras[i].y << '\n';
	}
	return 0;
}