#include<iostream>
using namespace std;

int cal(int n, int r){
	if(r == 0)
		return 1;
	return cal(n, r - 1) * (n - r + 1) / r;
}

int main(){
	int n, r;
	while(cin >> n >> r){
		if(n == 0 && r == 0)
			break;
		if(n < r){
			cout << "error!" <<endl;
			continue;
		}	
		int ans = cal(n, r);
		cout << ans << endl;
	}
	return 0;
}