#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <vector>
#include <queue>
#include <algorithm>
using namespace std;

#define Nmax 110
#define pb push_back
#define Inf 0x3f3f3f3f

int Cap[Nmax][Nmax], Flow[Nmax][Nmax], Source, Sink;
int N, DegIn, DegOut, M, Father[Nmax];
vector<int> G[Nmax];
bool Used[Nmax];

void Build_Graph()
{
    scanf("%i", &N);
    Source = 0;
    Sink = 2 * N + 1;
    for(int i = 1; i <= N; ++ i)
    {
        scanf("%i %i", &DegOut, &DegIn);
        Cap[Source][i] = DegOut;
        Cap[i + N][Sink] = DegIn;
        G[Source].pb(i); G[i].pb(Source);
        G[i + N].pb(Sink); G[Sink].pb(i + N);
        for(int j = 1; j <= N; ++ j)
            if(i != j)
            {
                Cap[i][j + N] = 1;
                G[i].pb(j + N); G[j + N].pb(i);
            }
    }
}

bool BFS()
{
    memset(Used, 0, sizeof(Used));
    queue<int> Q;
    Used[Source] = 1;
    Q.push(Source);
    while(!Q.empty())
    {
        int Node = Q.front(); Q.pop();
        if(Node == Sink) continue;
        for(vector<int> :: iterator it = G[Node].begin(); it != G[Node].end(); ++ it)
            if(!Used[*it] && Cap[Node][*it] > Flow[Node][*it])
            {
                Used[*it] = 1;
                Father[*it] = Node;
                Q.push(*it);
            }
    }
    return Used[Sink];
}

void MaxFlow()
{
    while(BFS())
    {
        for(vector<int> :: iterator it = G[Sink].begin(); it != G[Sink].end(); ++ it)
            if(Used[*it] && Cap[*it][Sink] != Flow[*it][Sink])
            {
                Father[Sink] = *it;
                int MinFlow = Inf;
                for(int Node = Sink; Node != Source; Node = Father[Node])
                    MinFlow = min(MinFlow, Cap[Father[Node]][Node] - Flow[Father[Node]][Node]);
                for(int Node = Sink; Node != Source; Node = Father[Node])
                {
                    Flow[Father[Node]][Node] += MinFlow;
                    Flow[Node][Father[Node]] -= MinFlow;
                }
            }
    }
}

void GetAns()
{
    int Ans = 0;
    for(int i = 1; i <= N; ++ i)
        for(int j = N + 1; j <= 2 * N; ++ j)
            if(Flow[i][j])
                Ans ++;
    printf("%i\n", Ans);
    for(int i = 1; i <= N; ++ i)
        for(int j = N + 1; j <= 2 * N; ++ j)
            if(Flow[i][j])
                printf("%i %i\n", i, j - N);
}

int main()
{
    freopen("harta.in", "r", stdin);
    freopen("harta.out", "w", stdout);
    Build_Graph();
    MaxFlow();
    GetAns();
    return 0;
}