#include <bits/stdc++.h>
using namespace std;
long long n, m, k, ans, fac[1010101], inv[1010101], pm[1010101], xxx[100],
    pn[1010101], mod = 1e9 + 7;
int a, b;
long long pmw(long long a, long long b) {
  long long ret = 1;
  if (b < 0) b += mod - 1;
  while (b) {
    if (b & 1) ret = ret * a % mod;
    a = a * a % mod;
    b /= 2;
  }
  return ret;
}
void input() {
  long long i, j;
  cin >> n >> m >> a >> b;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  input();
  long long i, j, temp = 0;
  pn[-1] = pmw(n, mod - 2);
  fac[0] = pm[0] = pn[0] = 1;
  for (i = 1; i < 1010101; i++) pm[i] = pm[i - 1] * m % mod;
  for (i = 1; i < 1010101; i++) pn[i] = pn[i - 1] * n % mod;
  for (i = 1; i < 1010101; i++) fac[i] = fac[i - 1] * i % mod;
  inv[1010100] = pmw(fac[1010100], mod - 2);
  for (i = 1010099; i >= 0; i--) inv[i] = inv[i + 1] * (i + 1) % mod;
  for (i = 1; i <= min(n - 1, m); i++) {
    temp = pm[n - 1 - i];
    temp = temp * (i + 1) % mod;
    temp = temp * pn[n - 2 - i] % mod;
    temp = temp * fac[m - 1] % mod;
    temp = temp * inv[i - 1] % mod;
    temp = temp * inv[m - i] % mod;
    temp = temp * fac[n - 2] % mod;
    temp = temp * inv[n - 1 - i] % mod;
    ans += temp;
    ans %= mod;
  }
  cout << ans;
}
