「10年OI一场空，看错模数见祖宗」

然后就是求C(n,m)，没别的了

#include<iostream>
using namespace std;
#define ll long long
const ll M = 1e8 + 7;
ll qpow(ll a, ll b){
    ll ans = 1, base = a;
    while (b){
        if (b&1) ans = ans * base % M;
        base = base * base % M;
        b >>= 1;
    }
    return ans;
}
int main(){
    ll n, m, c=1;
    cin >> n >> m;
    for (int i = 1;i <= m;++i){
        c = c * (n+1-i) % M;
        c = c * qpow(i, M-2) % M;
    }
    cout << c;
}