This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include<bits/stdc++.h>
using namespace std;
using ll = long long;
const int N=3005;
const int MOD=1e9+7;
int h,w;
ll dp[N][N];
int main(){
cin.tie(nullptr)->sync_with_stdio(false);
cin >> h >> w;
for(int i=0;i<=h;i++){
dp[i][0]=1;
}
for(int i=0;i<=w;i++){
dp[0][i]=1;
}
for(int i=1;i<=h;i++){
for(int j=1;j<=w;j++){
dp[i][j]+=dp[i-1][j];
dp[i][j]%=MOD;
dp[i][j]+=dp[i-1][j-1]*j*4;
dp[i][j]%=MOD;
if(i>=2){
dp[i][j]+=dp[i-2][j-1]*(i-1)*j%MOD;
dp[i][j]%=MOD;
}
if(j>=2){
dp[i][j]+=dp[i-1][j-2]*(1LL*j*(j-1)/2)%MOD;
dp[i][j]%=MOD;
}
}
}
cout << (dp[h][w]-1+MOD)%MOD;
}
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