#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7;
class {
 public:
  long long deg[1000009], INV[1000009];
  void solve() {
    init();
    long long n, m, a, b;
    cin >> n >> m >> a >> b;
    long long ans = 0;
    for (int i = 0; i <= m - 1 && i <= n - 2; i++) {
      long long tmp = deg[m - 1] % MOD * INV[i] % MOD * INV[m - i - 1] % MOD *
                      deg[n - 2] % MOD * INV[n - i - 2] % MOD *
                      pw(m, n - 2 - i) % MOD * pw(n, n - i - 3) % MOD *
                      (i + 2) % MOD;
      ans = (ans + tmp) % MOD;
    }
    cout << ans << endl;
  }
  long long inv(long long a) { return pw(a, MOD - 2); }
  void init() {
    deg[0] = 1;
    for (int i = 1; i < 1000009; i++) deg[i] = 1LL * deg[i - 1] * i % MOD;
    INV[1000009 - 1] = pw(deg[1000009 - 1], MOD - 2);
    for (int i = 1000009 - 2; i >= 0; i--) INV[i] = INV[i + 1] * (i + 1) % MOD;
  }
  int pw(long long a, long long b) {
    if (b < 0) b += MOD - 1;
    long long ret = 1;
    while (b) {
      if (b & 1) ret = ret * a % MOD;
      a = a * a % MOD;
      b >>= 1;
    }
    return ret;
  }
} NSPACE;
int main() {
  ios_base::sync_with_stdio(false);
  cout.tie(0);
  cin.tie(0);
  NSPACE.solve();
}
