#include<bits/stdc++.h>
using namespace std;
int n, m, a[20][20], maxn;
int h(int b, int c, int d, int e) {
	int sum = 0;
	for (int x = b; x <= d; x++) {
		for (int y = c; y <= e; y++) {
			if (a[x][y] == 1) {
				sum++;
			}
		}
	}
	return sum;
}
int b(int b2, int c, int d, int e) {
	int sum = 0;
	for (int x = b2; x <= d; x++) {
		for (int y = c; y <= e; y++) {
			if (a[x][y] == 0) {
				sum++;
			}
		}
	}
	return sum;
}
int main() {
	cin >> n >> m;
	for (int i = 1; i <= n; i++) {
		string s;
		cin >> s;
		for (int j = 0; j < m; j++) { //转换
			a[i][j + 1] = s[j] - '0';
		}
	}
	for (int x = 1; x <= n; x++) {
		for (int y = 1; y <= m; y++) {
			for (int x2 = x; x2 <= n; x2++) {
				for (int y2 = y; y2 <= m; y2++) {
					if (h(x, y, x2, y2) == b(x, y, x2, y2)) {
						maxn = max(maxn, h(x, y, x2, y2) + b(x, y, x2, y2));
					}
				}
			}
		}
	}
	cout << maxn;
	return 0;
}