#include<fstream>
#include<vector>
#define N 10010
using namespace std;

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

vector<int> v[N];
int n,ii[101],u[101],nr[N],nr22[N],nr1,nr2,l[N],r[N];
bool u[N];

bool pairup(int x) {
	
}

int cuplaj() {
	int i,c=0,ok=1;
	
	while(ok) {
		
		for(i=1;i<=nr1;++i)
			u[i]=false;
		
		ok=0;
		
		for(i=1;i<=n;++i) if(l[i]==0) 
			if(pairup(i)) {
				
				++c;
				ok=0;
			}
	}
	
	return c;
}

int main() {
	int i;
	
	in >> n;
	
	for(i=1;i<=n;++i) {
		in >> ii[i] >> o[i];
		
		v[++nr1].push_back(++nr2);
		
		nr[nr1]=i;
		nr22[nr2]=i;
	}
	
	out << cuplaj() << "\n";
	
	for(i=1;i<=nr1;++i)
		if(l[i])
			out << i << " " << l[i] << "\n";
	
	return 0;
}