#include <bits/stdc++.h>
using namespace std;
const int P = 1000000007;
long long power(long long x, long long t) {
  long long ret = 1;
  for (; t; t >>= 1, x = x * x % P) {
    if (t & 1) {
      ret = ret * x % P;
    }
  }
  return ret;
}
int main() {
  int n, m, a, b;
  cin >> n >> m >> a >> b;
  a--;
  b--;
  int N = max(n, m);
  vector<long long> fac(N + 1);
  fac[0] = 1;
  for (int i = 1; i <= N; ++i) {
    fac[i] = fac[i - 1] * i % P;
  }
  vector<long long> facinv(N + 1);
  facinv[N] = power(fac[N], P - 2);
  for (int i = N; i >= 1; --i) {
    facinv[i - 1] = facinv[i] * i % P;
  }
  auto C = [&](int n, int m) -> long long {
    if (n < m) {
      return 0LL;
    }
    return fac[n] * facinv[m] % P * facinv[n - m] % P;
  };
  vector<long long> pwm(n + 1);
  pwm[0] = 1;
  for (int i = 1; i <= n; ++i) {
    pwm[i] = pwm[i - 1] * m % P;
  }
  vector<long long> pwn(n + 1);
  pwn[0] = 1;
  for (int i = 1; i <= n; ++i) {
    pwn[i] = pwn[i - 1] * n % P;
  }
  long long ans = 0;
  for (int i = 2; i < n; ++i) {
    long long d = C(n - 2, i - 2) * i % P * pwn[n - i - 1] % P * fac[i - 2] %
                  P * C(m - 1, i - 2) % P * pwm[n - i] % P;
    ans += d;
    ans %= P;
  }
  ans += fac[n - 2] * C(m - 1, n - 2) % P;
  ans %= P;
  cout << ans << '\n';
  return 0;
}
