#include <bits/stdc++.h>
using namespace std;
const int N = 2500;
int arr[N][N];
int pref[N][N];
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
//if (n * n * n * m * m * m > 1e8) return 0;
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
char x; cin >> x;
arr[i][j] = x - '0';
pref[i][j] = x - '0';
pref[i][j] += (i ? pref[i - 1][j] : 0);
pref[i][j] += (j ? pref[i][j - 1] : 0);
pref[i][j] -= (i && j ? pref[i - 1][j - 1] : 0);
}
}
auto get = [&](int a, int b, int c, int d) ->int {
int s = pref[c][d];
s -= (a ? pref[a - 1][d] : 0);
s -= (b ? pref[c][b - 1] : 0);
s += (a && b ? pref[a - 1][b - 1] : 0);
return s;
};
// brute force
int Q = 2;
auto check = [&](int a, int b) ->int {
for (int i = 0; i + a - 1 < n; i++) {
for (int j = 0; j + b - 1 < m; j++) {
int g = get(i, j, i + a - 1, j + b - 1);
if (g == a * b) {
for (int i2 = i; i2 <= i + a - 1; i2++) {
for (int j2 = j; j2 <= j + b - 1; j2++)
arr[i2][j2] = Q;
}
}
}
}
int ok = 1;
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
if (arr[i][j] != 0 && arr[i][j] != Q) ok = 0;
}
}
Q++;
return ok;
};
int ans = 0;
for (int a = 1; a <= n; a++) {
int l = 1, r = m;
while (l < r) {
int b = (l + r + 1) / 2;
if (check(a, b))
l = b;
else
r = b - 1;
}
if (check(a, l))
ans = max(ans, a * l);
// for (int b = l; b <= r; b++) {
// if (check(a, b)) {
// ans = max(ans, a * b);
// }
// }
}
cout << ans << '\n';
return 0;
}
