#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
const double pi = acos(-1);
const int MOD = 1e9 + 7;
const int INF = 1e9 + 7;
const int MAXN = 1e6 + 5;
const double eps = 1e-9;
long long fac[MAXN], pwr[MAXN], pwm[MAXN];
long long pw(long long a, long long b) {
  long long ret = 1;
  while (b > 0) {
    if (b & 1)
      ret = (ret * a) % MOD, b--;
    else
      a = (a * a) % MOD, b /= 2;
  }
  return ret;
}
long long inv(long long a) { return pw(a, MOD - 2); }
long long ncr(long long n, long long k) {
  if (k > n) return 0;
  return (fac[n] * inv((fac[k] * fac[n - k]) % MOD)) % MOD;
}
int main() {
  int n, m, a, b;
  scanf("%d %d %d %d", &n, &m, &a, &b);
  fac[0] = 1;
  pwr[0] = 1;
  pwm[0] = 1;
  for (long long i = 1; i <= max(n, m); i++) {
    fac[i] = (fac[i - 1] * i) % MOD;
    pwr[i] = (1ll * pwr[i - 1] * (long long)n) % MOD;
    pwm[i] = (1ll * pwm[i - 1] * (long long)m) % MOD;
  }
  long long ans = 0;
  for (long long k = 1; k < min(m + 1, n); k++) {
    long long val = pwm[n - 1 - k];
    val = (val * fac[n - 2]) % MOD;
    val = (val * inv(fac[n - k - 1])) % MOD;
    if (k != n - 1) {
      val = (val * (1ll + k)) % MOD;
      val = (val * pwr[n - k - 2]) % MOD;
    }
    val = (val * ncr(m - 1, k - 1)) % MOD;
    ans = (ans + val) % MOD;
  }
  printf("%I64d\n", (ans));
  return 0;
}
