#include<iostream>
#include<deque>
#include<algorithm>
#include<vector>
#include<fstream>
#include<queue>
using namespace std;

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

struct edge{
	int a,b,capacity;
};

const int INF = 2e9;
const int mx = 230;
int n;
int num_nodes;
vector<int> ind,outd;
int capacity[mx][mx];
int flow[mx][mx];
vector<int> graph[mx];


void read(){
	in>>n;
	ind.resize(n+1);
	outd.resize(n+1);
	for(int i=1;i<=n;i++){
		in>>outd[i]>>ind[i];
	}
}

void add_edge(int a,int b,int cap){
	graph[a].push_back(b);
	graph[b].push_back(a);
	capacity[a][b]=cap;
}


int bfs(int s, int t, vector<int>& parent) {
    fill(parent.begin(), parent.end(), -1);
    parent[s] = -2;
    queue<pair<int, int>> q;
    q.push({s, INF});

    while (!q.empty()) {
        int cur = q.front().first;
        int flow = q.front().second;
        q.pop();

        for (int next : graph[cur]) {
            if (parent[next] == -1 && capacity[cur][next]) {
                parent[next] = cur;
                int new_flow = min(flow, capacity[cur][next]);
                if (next == t)
                    return new_flow;
                q.push({next, new_flow});
            }
        }
    }

    return 0;
}


int maxflow(int s, int t) {
    int flow = 0;
    vector<int> parent(mx);
    int new_flow;

    while ((new_flow = bfs(s, t, parent))) {
        flow += new_flow;
        int cur = t;
        while (cur != s) {
            int prev = parent[cur];
            capacity[prev][cur] -= new_flow;
            capacity[cur][prev] += new_flow;
            cur = prev;
        }
    }

    return flow;
}


void solve(){
	for(int i=1;i<=n;i++){
		add_edge(0,i,outd[i]);
	}	
	for(int i=1; i<=n;i++){
		for(int j = 1;j<=n;j++){
			if(i!=j){
				add_edge(i,n+j,1);
			}
		}
	}
	for(int j=1;j<=n;j++){
		add_edge(n+j,2*n+1,ind[j]);
	}
	int start=0,end=2*n+1;
	out<<maxflow(start,end)<<endl;
	for(int i=1;i<=n;i++){
		for(int j=1;j<=n;j++){
			if(i==j)
				continue;
			if(capacity[i][n+j]==0){
				out<<i<<" "<<j<<endl;
			}
		}
	}

}



int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	read();
	solve();

	return 0;
}