#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
int main() {
int n, m;
cin >> n >> m;
vector<string> s(n);
for (int i = 0; i < n; ++i) {
cin >> s[i];
}
vector<vector<int>> pr(n + 1, vector<int>(m + 1));
for (int i = 0; i < n; ++i) {
for (int j = 0; j < m; ++j) {
int val = s[i][j] - '0';
pr[i + 1][j + 1] = val + pr[i + 1][j] + pr[i][j + 1];
}
}
int maxlen = n;
for (int j = 0; j < m; ++j) {
int L0 = 0;
for (int i = 0; i < n; ++i) {
if (s[i][j] == '1') {
L0++;
}
else {
if (L0) {
maxlen = min(maxlen, L0);
}
L0 = 0;
}
}
if (L0) {
maxlen = min(maxlen, L0);
}
}
int tl = 0, tr = m;
while (tl < tr - 1) {
int tm = (tl + tr) >> 1;
bool ok = true;
for (int i = 0; i < n - maxlen; ++i) {
if (!ok) break;
for (int j = 0; j < m - tm; ++j) {
int S = pr[i + maxlen][j + tm] + pr[i][j] - pr[i + maxlen][j] - pr[i][j + tm];
if (S != (maxlen * tm)) {
ok = false;
break;
}
}
}
if (ok) {
tl = tm;
}
else tr = tm;
}
cout << (maxlen * tr);
}
