#include <cstdio>
#include <string>
#define maxn 256

int c[maxn][maxn], n, source, sink;

void citire()
{
	freopen("harta.in", "r", stdin);
	scanf("%d\n", &n);
	int p, q;
	sink=2*n+1;
	source=0;
	
	for(int i=1;i<=n;i++)
	{
		scanf("%d %d\n", &p, &q);
		
		c[source][i]=p;
		c[i+n][sink]=q;
	}

	for(int i=1;i<=n;i++)
		for(int j=n+1;j<=n<<1;j++) c[i][j]=1;
		
	for(int i=1;i<=n;i++) c[i][i+n]=0;
}

int bfs()
{
	int Q[maxn], t[maxn], viz[maxn], p=1, q=1, u, i;
	memset(t, -1, sizeof(t));
	memset(viz, 0, sizeof(viz));
	Q[1]=source;
	viz[source]=1;
	
	while(p<=q)
	{
		u=Q[p++];
		
		for(i=source;i<=sink;i++)
			if(c[u][i] && !viz[i])
			{
				viz[i]=1;
				t[i]=u;
				Q[++q]=i;
				if(i==source) goto exit_loop;
			}
	}
	exit_loop:
	

	if(t[sink]==-1) return 0;
	
	
	for(i=sink; i; i=t[i])
	{
		c[t[i]][i]-=1;
		c[i][t[i]]+=1;
	}

	
	return 1;
}


void max_flow()
{
	while(bfs());
	
	int i, j, nr=0;
	
	for(i=1;i<=n;i++)
		for(j=n+1;j<=n<<1;j++) if(!c[i][j] && i!=j-n) nr++;
	
	freopen("harta.out", "w", stdout);
	printf("%d\n", nr);
	
	for(i=1;i<=n;i++)
		for(j=n+1;j<=n<<1;j++) if(!c[i][j] && i!=j-n) printf("%d %d\n", i, j-n);
	
		
}


int main()
{

	citire();
	max_flow();
	return 0;
}