#include <bits/stdc++.h>
using namespace std;
const long long mod = (int)1e9 + 7;
const int maxn = 1000005;
long long fac[maxn], inv_fac[maxn];
long long quick(long long a, long long b) {
  long long ans = 1;
  while (b) {
    if (b & 1) ans = ans * a % mod;
    a = a * a % mod;
    b /= 2;
  }
  return ans;
}
void init() {
  inv_fac[0] = fac[0] = 1, fac[1] = inv_fac[1] = 1;
  for (int i = 2; i < maxn; i++) fac[i] = fac[i - 1] * i % mod;
  inv_fac[maxn - 1] = quick(fac[maxn - 1], mod - 2);
  for (int i = maxn - 2; i >= 0; i--)
    inv_fac[i] = inv_fac[i + 1] * (i + 1) % mod;
}
long long C(long long n, long long m) {
  if (n < 0 || m < 0 || m > n) return 0;
  if (n == m || m == 0) return 1;
  return fac[n] * inv_fac[n - m] % mod * inv_fac[m] % mod;
}
int n, m, a, b;
int main() {
  init();
  scanf("%d%d%d%d", &n, &m, &a, &b);
  long long ans = 0;
  for (int i = 0; i <= n - 2; i++) {
    if (i >= m) break;
    if (i == n - 2)
      ans = (ans + C(m - 1, i) * fac[i] % mod) % mod;
    else
      ans = (ans + C(n - 2, i) * fac[i] % mod * C(m - 1, i) % mod *
                       quick(m, n - 2 - i) % mod * (2 + i) % mod *
                       quick(n, n - (i + 2) - 1)) %
            mod;
  }
  printf("%lld\n", ans);
  return 0;
}
