Combination

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
ll fact[200005];
ll mod = 1e9 + 7;
ll mul(ll x, ll y) { return (x * y) % mod; }
ll pw(ll x, ll y) {
    ll z = 1;
    while (y) {
        if (y & 1) z = mul(z, x);
        x = mul(x, x);
        y >>= 1;
    }
    return z;
}
ll inv(ll x, ll mod) { return pw(x, mod - 2); }
ll nCk(ll n, ll k, ll p) {
    if (n < k)return 0;
    return ((fact[n] * inv(fact[k], p) % p) * inv(fact[n - k], p)) % p;
}
int main() {
    fact[0] = 1;
    for (int i = 1; i <= 200002; i++) {
        fact[i] = mul(fact[i - 1], i);
    }

    cout << nCk(5, 2, mod); //5 C 2, mod로 나눈 나머지
    return 0;
}