#include<iostream>

using namespace std;

int a[4001][4001],tem;

int n,m,ans,i,j,k,l;

int main()

{

scanf("%d%d",&m,&n);

for (i=1;i<=m;++i)

{

char s[10000];

gets(s);

for (j=1;j<=n;++j)

if (s[j-1]=='0') a[i][j]=-1;

else a[i][j]=1;

}

int b[4001]={0};

for (i=1;i<=m;++i)

{

memset(b,0,sizeof(b));

for (j=i;j<=m;++j)

{

for (k=1;k<=n;++k)

b[k]+=a[j][k];

int now=0;

for (k=1;k<=n;++k)

now=max(now+b[k],b[k]);

if (now>ans) ans=now;

}

}

printf("%d\n",ans);

return 0;

}