求解T1

RT,为什么我这么写前缀和会错?

#include<bits/stdc++.h>
using namespace std;
int a[5010][5010];
int s[5010][5010];
bool b[5010];
template<typename T>void qin(T &x)
{
    x=0;bool f=0;
    char ch=getchar();
    while(!isdigit(ch)&&ch!='-') ch=getchar();
    if(ch=='-') ch=getchar(),f=1;
    while(isdigit(ch)) x=x*10+ch-'0',ch=getchar();
    if(f==1) x=-x;
}
int main()
{
    int n,m;
    qin(n),qin(m);
    int i,j;
    for(i=1;i<=n;i++)
        for(j=1;j<=m;j++){
            qin(a[i][j]);
            s[i][j]=s[i][j-1]+a[i][j];
        }
    int c=0x7ffffff,k=0x7ffffff;
    for(i=1;i<=m;i++){
        int c1=0;
        for(j=1;j<=n;j++)
            if(s[j][i]==i){
                b[i]=1;
                c1++;
            }
        if(!b[i]){
            k=i-1;
            break;
        }
        c=min(c1,c);
    }
    cout<<c*k;
    return 0;
}