I was solving this question Grid Game - Prefix Sum Problem | CodeChef and when I tried to run my program I was getting this error

**prog.cpp: In function ‘int main()’:
prog.cpp:30:26: error: invalid types ‘long long int[long long int]’ for array subscript
sum+=a[i][j];**

here is my Program

‘’’

#include

using namespace std;

int main() {

// your code goes here

int t;

cin>>t;

while(t–){

long long n,m;

cin>>n>>m;

long long a[n][m];

for(long long i=0;i<n;i++){

for(long long j=0;j<m;j++){

cin>>a[i][j];

}

}

long long query;

cin>>query;

```
while(query--){
long long a,b,x,y;
cin>>a>>b>>x>>y;
a--;
b--;
x--;
y--;
for(long long i=a;i<=x;i++){
long long sum=0;
for(long long j=b;j<=y;j++){
sum+=a[i][j];
}
cout<<sum<<endl;
}
}
}
return 0;
```

}

‘’’

can someone please tell me what am I doing wrong and what this error means?

(I tried to formate my code but it’s not working ProPerly)