#include <bits/stdc++.h>
using namespace std;
template <typename T>
void read(T &x) {
  x = 0;
  char ch = getchar();
  long long f = 1;
  while (!isdigit(ch)) {
    if (ch == '-') f *= -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = x * 10 + ch - 48;
    ch = getchar();
  }
  x *= f;
}
int mod = 1000000007;
inline int mul(int x, int y) { return 1ll * x * y % mod; }
int add(int x, int y) { return x + y >= mod ? x + y - mod : x + y; }
int sub(int x, int y) { return x - y < 0 ? x - y + mod : x - y; }
int sq(int x) { return 1ll * x * x % mod; }
int pow(int a, int b) {
  return b <= 0 ? 1 : (b & 1 ? mul(a, sq(pow(a, b / 2))) : sq(pow(a, b / 2)));
}
int n, m, a, b;
const int N = 1000010;
int fac[N], invfac[N], pown[N], powm[N];
int C(int n, int m) {
  return 1ll * fac[n] * (1ll * invfac[m] * invfac[n - m] % mod) % mod;
}
int F(int n, int m) {
  if (n == m) return 1;
  return 1ll * pown[n - m - 1] * m % mod;
}
int main() {
  cin >> n >> m >> a >> b;
  fac[0] = pown[0] = powm[0] = 1;
  for (int i = 1; i < N; i++) {
    fac[i] = 1ll * fac[i - 1] * i % mod;
    pown[i] = 1ll * pown[i - 1] * n % mod;
    powm[i] = 1ll * powm[i - 1] * m % mod;
  }
  invfac[N - 1] = pow(fac[N - 1], mod - 2);
  for (int i = N - 2; i >= 0; i--) {
    invfac[i] = 1ll * invfac[i + 1] * (i + 1) % mod;
  }
  int ans = 0;
  for (int i = 1; i <= m; i++) {
    if (i + 1 > n) break;
    int bsel = mul(C(m - 1, i - 1), powm[n - 1 - i]);
    int tsel = mul(fac[i - 1], mul(C(n - 2, i - 1), F(n, i + 1)));
    ans = add(ans, mul(bsel, tsel));
  }
  cout << ans << endl;
  return 0;
}
