#include <bits/stdc++.h>
using namespace std;
inline long long mod(long long n, long long m) {
  long long ret = n % m;
  if (ret < 0) ret += m;
  return ret;
}
long long gcd(long long a, long long b) {
  return (b == 0LL ? a : gcd(b, a % b));
}
long long exp(long long a, long long b, long long m) {
  if (b == 0LL) return 1LL;
  if (b == 1LL) return mod(a, m);
  long long k = mod(exp(a, b / 2, m), m);
  if (b & 1LL) {
    return mod(a * mod(k * k, m), m);
  } else
    return mod(k * k, m);
}
const int N = 1e6 + 10;
long long fat[N], inv[N];
const int M = 1e9 + 7;
long long C(int n, int k) {
  if (k > n) return 0;
  return fat[n] * inv[k] % M * inv[n - k] % M;
}
long long pwr[N];
long long Caley(int x, int y) {
  if (x == y) return 1;
  return y * exp(x, x - y - 1, M) % M;
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  fat[0] = inv[0] = 1;
  for (int i = 1; i < N; i++) fat[i] = i * fat[i - 1] % M;
  inv[N - 1] = exp(fat[N - 1], M - 2, M);
  for (int i = N - 2; i >= 0; i--) {
    inv[i] = inv[i + 1] * (i + 1) % M;
  }
  int n, m, a, b;
  cin >> n >> m >> a >> b;
  pwr[0] = 1;
  for (int i = 1; i < N; i++) {
    pwr[i] = pwr[i - 1] * m % M;
  }
  long long tot = exp(n + 1, n - 1, M);
  long long ans = 0;
  for (int v = 2; v <= n; v++) {
    if (v - 2 > m - 1) break;
    long long stars = C(m - 1, v - 2);
    long long trees = Caley(n, v);
    long long path = C(n - 2, v - 2) * fat[v - 2] % M;
    long long wh = pwr[n - v];
    ans += (stars * trees % M * path % M * wh % M);
    if (ans >= M) ans -= M;
  }
  cout << ans << "\n";
}
