#include <bits/stdc++.h>
using namespace std;
const int maxn = 1000010;
const int mod = 1000000007;
const int N = 1000000;
int p[maxn], inv[maxn];
int n, m, ans;
int pow(int a, int b) {
  int ret = 1;
  for (; b; b >>= 1) {
    if (b & 1) ret = 1ll * ret * a % mod;
    a = 1ll * a * a % mod;
  }
  return ret;
}
int C(int n, int k) {
  if (n < k) return 0;
  return 1ll * p[n] * inv[k] % mod * inv[n - k] % mod;
}
int main() {
  cin >> n >> m;
  p[0] = 1;
  for (int i = 1; i <= N; i++) {
    p[i] = 1ll * p[i - 1] * i % mod;
  }
  inv[N] = pow(p[N], mod - 2);
  for (int i = N - 1; i >= 0; i--) {
    inv[i] = 1ll * inv[i + 1] * (i + 1ll) % mod;
  }
  for (int i = 0; i <= n - 2; i++) {
    int now = 1ll * C(n - 2, i) * p[i] % mod * C(m - 1, i) % mod;
    now = 1ll * pow(m, n - 2 - i) * now % mod;
    if (n - i - 3 >= 0) {
      now = 1ll * now * (i + 2) % mod * pow(n, n - i - 3) % mod;
    }
    ans = (ans + now) % mod;
  }
  cout << ans << endl;
  return 0;
}
