#include <bits/stdc++.h>
using namespace std;
const int mo = 1e9 + 7;
long long ksm(long long x, long long y) {
  long long s = 1;
  if (y < 0) x = ksm(x, mo - 2), y = -y;
  for (; y; y /= 2, x = x * x % mo)
    if (y & 1) s = s * x % mo;
  return s;
}
const int N = 1e6 + 5;
int n, m, a, b;
long long fac[N], nf[N], ans;
long long P(int n, int m) {
  if (n < m) return 0;
  return fac[n] * nf[n - m] % mo;
}
long long C(int n, int m) {
  if (n < m) return 0;
  return fac[n] * nf[m] % mo * nf[n - m] % mo;
}
int main() {
  n = 1e6;
  fac[0] = 1;
  for (int i = 1; i <= n; i++) fac[i] = fac[i - 1] * i % mo;
  nf[n] = ksm(fac[n], mo - 2);
  for (int i = n; i >= 1; i--) nf[i - 1] = nf[i] * i % mo;
  scanf("%d %d %d %d", &n, &m, &a, &b);
  for (int x = 1; x <= n - 1; x++)
    ans += P(n - 2, x - 1) * C(m - 1, x - 1) % mo * ksm(m, n - 1 - x) % mo *
           ksm(n, n - 1 - (x + 1)) % mo * (x + 1) % mo;
  printf("%I64d", ans % mo);
}
