#include <bits/stdc++.h>
using namespace std;
void pr_init() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
}
long long fact[1000001];
long long inv_fact[1000001];
long long raise(long long x, long long y) {
  long long res = 1;
  while (y > 0) {
    if (y % 2 != 0) res = (res % 1000000007 * x % 1000000007) % 1000000007;
    y /= 2;
    x = (x % 1000000007 * x % 1000000007) % 1000000007;
  }
  return res;
}
void precompute() {
  memset(fact, 0, sizeof(fact));
  memset(inv_fact, 0, sizeof(inv_fact));
  fact[2] = 2, fact[1] = 1, fact[0] = 1;
  for (long long i = 3; i < 1000001; i++)
    fact[i] = (i % 1000000007 * fact[i - 1] % 1000000007) % 1000000007;
  for (long long i = 1; i < 1000001; i++)
    inv_fact[i] = raise(fact[i], 1000000007 - 2) % 1000000007;
  inv_fact[0] = 1;
}
long long comb(long long n, long long r) {
  if (r == 0 || n == r) return 1;
  if (r > n || r < 0) return 0;
  long long res = 1;
  res = (fact[n] % 1000000007 * inv_fact[r] % 1000000007) % 1000000007;
  res = (res % 1000000007 * inv_fact[n - r] % 1000000007) % 1000000007;
  return res;
}
long long perm(long long n, long long r) {
  if (r == 0) return 1;
  if (r < 0 || r > n) return 0;
  return (fact[n] % 1000000007 * inv_fact[n - r] % 1000000007) % 1000000007;
}
void solve() {
  precompute();
  long long n, m, a, b;
  cin >> n >> m >> a >> b;
  long long res = 0;
  for (long long k = 1; k < n; k++) {
    long long ff = 1;
    ff = (perm(n - 2, k - 1) % 1000000007 * comb(m - 1, k - 1) % 1000000007) %
         1000000007;
    ff = (ff % 1000000007 * (k + 1) % 1000000007) % 1000000007;
    if (n - k - 1 >= 0)
      ff = (ff % 1000000007 * raise(m, n - k - 1) % 1000000007) % 1000000007;
    else
      ff = (ff % 1000000007 * raise(raise(m, fabs(n - k - 1)), m - 2) %
            1000000007) %
           1000000007;
    if (n - k - 2 >= 0)
      ff = (ff % 1000000007 * raise(n, n - k - 2) % 1000000007) % 1000000007;
    else {
      ff = (ff % 1000000007 * raise(raise(n, fabs(n - k - 2)), 1000000007 - 2) %
            1000000007) %
           1000000007;
    }
    res += ff;
    res %= 1000000007;
  }
  cout << res << endl;
}
int32_t main() {
  solve();
  return 0;
}
