#include <bits/stdc++.h>
using namespace std;
long long pow_mod(long long x, long long n) {
  if (n == -1) return pow_mod(x, 1000000007 - 2);
  long long ret = 1;
  while (n > 0) {
    if (n & 1) ret = ret * x % 1000000007;
    x = x * x % 1000000007;
    n >>= 1;
  }
  return ret;
}
long long n, m, a, b, fac[1000005], inv[1000005];
long long C(long long up, long long down) {
  return fac[down] * inv[up] % 1000000007 * inv[down - up] % 1000000007;
}
int main() {
  fac[0] = 1;
  for (int i = 1; i <= 1000000; i++) fac[i] = fac[i - 1] * i % 1000000007;
  for (int i = 0; i <= 1000000; i++) inv[i] = pow_mod(fac[i], 1000000007 - 2);
  scanf("%lld%lld%lld%lld", &n, &m, &a, &b);
  long long sum = 0;
  for (long long i = 0; i + 2 <= n && i + 1 <= m; i++) {
    long long res = C(i, m - 1) * C(i, n - 2) % 1000000007 * fac[i] %
                    1000000007 * (i + 2) % 1000000007 * pow_mod(n, n - i - 3) %
                    1000000007 * pow_mod(m, n - i - 2) % 1000000007;
    sum = (sum + res) % 1000000007;
  }
  printf("%lld\n", sum);
  return 0;
}
