样例输出非常离谱

#include <bits/stdc++.h>
using namespace std;
int main(){
    int n,m;
    cin>>n>>m;
    int a[n+1][m+1]={};
    char d;
    for(int i=1;i<=n;i++){
        for(int j=1;j<=m;j++){
            cin>>d;
            a[i][j]=d-'0';
        }
    }
    int b[n+1][m+1]={};
    for(int i=1;i<=n;i++){
        for(int j=1;j<=m;j++){
            b[i][j]=a[i][j]+b[i-1][j]+b[i][j-1]-b[i-1][j-1];
        }
    }
    int max1=0;
    for(int c=1;c<=min(n,m);c++){
        for(int i=1;i<=n-c+1;i++){
            for(int j=1;j<=m-c+1;j++){
                int i1=i+c,j1=j+c;
                int s=b[i1][j1]-b[i-1][j1]-b[i1][j-1]+b[i][j];
                max1=max(max1,s);
            }
        }
    }
    cout<<max1;
    return 0;
}