入门题解

#include<bits/stdc++.h>
using namespace std;
#define int long long
signed main(){
	int n,m,p=1;
	cin>>n>>m;
	n--;
	m--;
	for(int i=1; i<=m; i++){
		p*=n-i+1;
		p/=i;
	}
    if(m<0||m>n) cout<<0;
    else cout<<p;
}

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