#include <bits/stdc++.h>
using namespace std;
void err(istream_iterator<string> it) { cerr << endl; }
template <typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << " = " << a << " , ";
  err(++it, args...);
}
namespace fastIO {
bool IOerror = 0;
inline char nc() {
  static char buf[100000], *p1 = buf + 100000, *pend = buf + 100000;
  if (p1 == pend) {
    p1 = buf;
    pend = buf + fread(buf, 1, 100000, stdin);
    if (pend == p1) {
      IOerror = 1;
      return -1;
    }
  }
  return *p1++;
}
inline bool blank(char ch) {
  return ch == ' ' || ch == '\n' || ch == '\r' || ch == '\t';
}
inline void read(int &x) {
  char ch;
  while (blank(ch = nc()))
    ;
  if (IOerror) return;
  for (x = ch - '0'; (ch = nc()) >= '0' && ch <= '9'; x = x * 10 + ch - '0')
    ;
}
}  // namespace fastIO
using namespace fastIO;
const double PI = acos(-1.0);
const double eps = 1e-8;
const int maxn = 2e6 + 7;
const long long mod = 1e9 + 7;
long long fac[maxn], finv[maxn];
long long qp(long long b, long long n) {
  long long r = 1;
  if (n <= 0) n += mod - 1;
  b %= mod;
  for (; n; n >>= 1, b = b * b % mod)
    if (n & 1) r = r * b % mod;
  return r;
}
long long pw[maxn];
void init() {
  fac[1] = fac[0] = finv[0] = 1;
  for (int i = 2; i < maxn; ++i) fac[i] = fac[i - 1] * i % mod;
  finv[maxn - 1] = qp(fac[maxn - 1], mod - 2);
  for (int i = maxn - 2; i >= 1; --i) finv[i] = finv[i + 1] * (i + 1) % mod;
}
long long C(int n, int m) {
  return n >= m ? fac[n] * finv[n - m] % mod * finv[m] % mod : 0;
}
long long A(int n, int m) { return n >= m ? fac[n] * finv[n - m] % mod : 0; }
int main() {
  int n, m, a, b;
  cin >> n >> m >> a >> b;
  init();
  long long res = 0;
  pw[0] = 1;
  for (int i = 1; i < maxn; ++i) pw[i] = pw[i - 1] * m % mod;
  for (int i = 0; i <= n - 2; ++i) {
    long long tmp = A(n - 2, i) * C(m - 1, i) % mod * pw[n - 2 - i] % mod *
                    (i + 2) % mod * qp(n, n - i - 3) % mod;
    res = (res + tmp) % mod;
  }
  cout << res << endl;
}
