#include <bits/stdc++.h>
using namespace std;
const int OO = 1e9;
const double EPS = 1e-9;
template <class T>
void _db(const char* dbStr, T e) {
  cout << dbStr << " = " << e << endl;
}
template <class T, class... L>
void _db(const char* dbStr, T e, L... r) {
  while (*dbStr != ',') cout << *dbStr++;
  cout << " = " << e << ',';
  _db(dbStr + 1, r...);
}
template <class S, class T>
ostream& operator<<(ostream& o, const map<S, T>& v) {
  o << "[";
  int i = 0;
  for (const pair<S, T>& pr : v)
    o << (!i++ ? "" : ", ") << "{" << pr.first << " : " << pr.second << "}";
  return o << "]";
}
template <template <class, class...> class S, class T, class... L>
ostream& operator<<(ostream& o, const S<T, L...>& v) {
  o << "[";
  int i = 0;
  for (const auto& e : v) o << (!i++ ? "" : ", ") << e;
  return o << "]";
}
template <class S, class T>
ostream& operator<<(ostream& o, const pair<S, T>& pr) {
  return o << "(" << pr.first << ", " << pr.second << ")";
}
ostream& operator<<(ostream& o, const string& s) {
  for (const char& c : s) o << c;
  return o;
}
template <class T>
using V = vector<T>;
template <class T>
using VV = V<V<T>>;
template <class T>
using VVV = VV<V<T>>;
using ll = long long;
using pii = pair<int, int>;
using vi = V<int>;
using vii = V<pii>;
using vvi = VV<int>;
using mii = map<int, int>;
using umii = unordered_map<int, int>;
using si = set<int>;
using usi = unordered_set<int>;
const int MOD = 1000000007;
ll power(ll num, ll exp) {
  ll ret = 1;
  while (exp) {
    if (exp & 1) ret = (ret * num) % MOD;
    num = (num * num) % MOD;
    exp >>= 1;
  }
  return ret;
}
ll add(ll x, ll y) {
  x += y;
  while (x >= MOD) x -= MOD;
  while (x < 0) x += MOD;
  return x;
}
ll mul(ll x, ll y) { return (x * y) % MOD; }
ll inv(ll a, ll b) { return 1 < a ? b - inv(b % a, a) * b / a : 1; }
const int MX = 2e6 + 5;
int fact[MX], factInv[MX];
int main() {
  ios::sync_with_stdio(false);
  cout.precision(10);
  cin.tie(0);
  fact[0] = 1;
  factInv[0] = inv(1, MOD);
  for (int i = 1; i < MX; ++i) {
    fact[i] = mul(fact[i - 1], i);
    factInv[i] = inv(fact[i], MOD);
  }
  int n, m, a, b;
  cin >> n >> m >> a >> b;
  int nPi = 1;
  int ans = 0;
  for (int i = 1; i <= min(n - 1, m); ++i) {
    true;
    int toAdd = mul(nPi, power(m, n - 1 - i));
    true;
    int ch = mul(fact[m - 1], mul(factInv[i - 1], factInv[m - i]));
    true;
    toAdd = mul(toAdd, ch);
    true;
    if (i != n - 1) {
      toAdd = mul(toAdd, i + 1);
      toAdd = mul(toAdd, power(n, n - i - 2));
    }
    true;
    ans = add(ans, toAdd);
    nPi = mul(nPi, n - i - 1);
    true;
  }
  cout << ans << '\n';
  return 0;
}
