func_code_string: #include <bits/stdc++.h>   using namespace std;   const long long mod = 1000000007;   int n;   int m;   int a, b;   long long fast_mod_pow(long long a, long long b) {     long long res = 1;     a %= mod;     while (b) {       if (b & 1) {         res = (res * a) % mod;       }       a = (a * a) % mod;       b >>= 1;     }     return res;   }   const int MAXN = 1000005;   long long fac[MAXN];   long long invfact[MAXN];   int main() {     cin >> n >> m >> a >> b;     long long ans = 0;     fac[0] = 1;     fac[1] = 1;     for (int i = 2; i < MAXN; i++) {       fac[i] = fac[i - 1] * i;       fac[i] %= mod;     }     invfact[MAXN - 1] = fast_mod_pow(fac[MAXN - 1], mod - 2);     for (int i = MAXN - 2; i >= 0; i--) {       invfact[i] = (invfact[i + 1] * (i + 1)) % mod;     }     for (int i = 1; i <= m; i++) {       if (i > n - 1) break;       long long temp = 1;       temp *= fac[m - 1];       temp %= mod;       temp *= invfact[m - i];       temp %= mod;       temp *= invfact[i - 1];       temp %= mod;       temp *= fac[n - 2];       temp %= mod;       temp *= invfact[n - i - 1];       temp %= mod;       if ((n - i - 1) == 0) {         ans += temp;         ans %= mod;         continue;       }       temp *= fast_mod_pow((long long)m, (long long)(n - i - 1));       temp %= mod;       temp *= (i + 1);       temp %= mod;       temp *= fast_mod_pow((long long)n, (long long)(n - i - 2));       temp %= mod;       ans += temp;       ans %= mod;     }     cout << ans << endl;     return 0;   }   