#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
const int maxn = 1000000 + 5;
int f[maxn], inv[maxn], finv[maxn];
void init() {
  inv[1] = 1;
  for (int i = 2; i < maxn; i++)
    inv[i] = (mod - mod / i) * 1ll * inv[mod % i] % mod;
  f[0] = finv[0] = 1;
  for (int i = 1; i < maxn; i++) {
    f[i] = f[i - 1] * 1ll * i % mod;
    finv[i] = finv[i - 1] * 1ll * inv[i] % mod;
  }
}
int C(int n, int m) {
  if (m < 0 || m > n) return 0;
  return f[n] * 1ll * finv[n - m] % mod * finv[m] % mod;
}
long long qpow(long long x, long long n) {
  long long r = 1;
  while (n > 0) {
    if (n & 1) r = r * x % mod;
    n >>= 1;
    x = x * x % mod;
  }
  return r;
}
long long T(long long n, long long k) {
  if (k == n) return 1;
  return k * qpow(n, n - k - 1) % mod;
}
int n, m, a, b;
int main() {
  init();
  cin >> n >> m >> a >> b;
  long long ans = 0;
  for (int i = 1; i < n; i++) {
    long long tot = 1ll * C(m - 1, i - 1) * C(n - 2, i - 1) % mod * f[i - 1] %
                    mod * qpow(m, n - i - 1) % mod * T(n, i + 1) % mod;
    ans += tot;
    if (ans >= mod) ans -= mod;
  }
  cout << ans << endl;
  return 0;
}
