#include <bits/stdc++.h>
using namespace std;
int n, m, u, v;
const int maxn = 1e6 + 10;
const int mod = 1e9 + 7;
const int N = 1e6 + 5;
long long inv[N];
void get_inverse(int n, int p) {
  inv[1] = 1;
  for (int i = 2; i <= n; ++i) {
    inv[i] = (long long)(p - p / i) * inv[p % i] % p;
  }
}
long long cmk[maxn], f[maxn], mi[maxn], fr[maxn];
namespace MI {
void A(long long &a, long long b) {
  a += b;
  if (a >= mod) a -= mod;
}
long long qmul(long long a, long long b) {
  long long res = 0;
  while (b) {
    if (b & 1) A(res, a);
    A(a, a);
    b >>= 1;
  }
  return res;
}
long long qpow(long long a, long long b) {
  long long res = 1;
  while (b) {
    if (b & 1) res = res * a % mod;
    a = a * a % mod;
    b >>= 1;
  }
  return res;
}
};  // namespace MI
signed main() {
  cin >> n >> m >> u >> v;
  n--;
  f[n] = 1;
  f[0] = 1;
  get_inverse(n, mod);
  for (int i = n - 1; i > 0; i--) {
    f[i] = f[i + 1] * i % mod;
  }
  cmk[0] = 1;
  for (int i = 1; i <= min(n, m); i++) {
    cmk[i] = cmk[i - 1] * (m - i) % mod * inv[i] % mod;
  }
  mi[0] = 1;
  for (int i = 1; i <= n; i++) {
    mi[i] = mi[i - 1] * m % mod;
  }
  for (int i = 1; i < n; i++) {
    fr[i] = (i + 1) * MI::qpow(n + 1, n - i - 1) % mod;
  }
  fr[n] = 1;
  long long ans = 0;
  for (int i = 1; i <= n; i++) {
    ans += cmk[i - 1] * f[n - i + 1] % mod * mi[n - i] % mod * fr[i] % mod;
    if (ans >= mod) ans -= mod;
  }
  cout << ans << endl;
  return 0;
}
