#include<cstdio>

#include<cstring>

#include<cctype>

#include<algorithm>

using namespace std;

#define rep(i,s,t) for(int i=s;i<=t;i++)

#define dwn(i,s,t) for(int i=s;i>=t;i--)

int read(){

	int x=0;char c=getchar();

	while(!isdigit(c)) c=getchar();

	while(isdigit(c)) x=x*10+c-'0',c=getchar();

	return x;

}

const int nmax=505;

int a[nmax][nmax];

void maxs(int &a,int b){

	if(a<b) a=b;

}

int main(){

	int m=read(),n=read();

	rep(i,1,n) rep(j,1,m) a[i][j]=read(),a[i][j]+=a[i-1][j];

	int ans=0,cnt;

	rep(i,1,n) rep(j,i,n) {

		cnt=0;

		rep(k,1,m){

		    if(a[j][k]-a[i-1][k]!=j-i+1) maxs(ans,cnt*(j-i+1)),cnt=0;

		    else cnt++;

	    }

	    maxs(ans,cnt*(j-i+1));

	}

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

	return 0;

}