#include "bits/stdc++.h" using namespace std; #ifdef DEBUG auto&operator<<(auto &o, pair<auto, auto> p) {o << "(" << p.first << ", " << p.second << ")"; return o;} auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{"; for(auto e : x) o<<e<<", "; return o<<"}";} #define debug(X) cerr << "["#X"]: " << X << '\n'; #else #define cerr if(0)cout #define debug(X) ; #endif using ll = long long; #define all(v) (v).begin(), (v).end() #define ssize(x) int(x.size()) #define fi first #define se second #define mp make_pair #define eb emplace_back const int mod = 1'000'000'007; int add(int a, int b) { a += b; return a >= mod ? a-mod : a; } int sub(int a, int b) { return add(a, mod-b); } int mul(int a, int b) { return int(((ll)a*b)%mod); } int fpow(int a, int b) { int res = 1; while (b) { if (b&1) { res = mul(a, res); b--; } a = mul(a, a); b /= 2; } return res; } int inv(int x) { return fpow(x, mod-2); } int main() { ios_base::sync_with_stdio(false); cin.tie(nullptr); int n, m; cin >> n >> m; vector<vector<int>> dp(n+1, vector<int>(m+1, 1)); for (int i = 1; i <= n; ++i) { for (int j = 1; j <= m; ++j) { dp[i][j] = 0; if (i-2 >= 0 && j-1 >= 0) dp[i][j] = add(dp[i][j], mul(mul(i, mul(i-1, inv(2))), dp[i-2][j-1])); if (i-1 >= 0 && j-2 >= 0) dp[i][j] = add(dp[i][j], mul(i, mul(j-1, dp[i-1][j-2]))); dp[i][j] = add(dp[i][j], mul(4*i, dp[i-1][j-1])); dp[i][j] = add(dp[i][j], dp[i][j-1]); } } debug(dp); cout << sub(dp[n][m], 1) << '\n'; return 0; }