#include <bits/stdc++.h>
using namespace std;
const int N = 2500;
int arr[N][N];
int pref[N][N];
class segment_tree {
public:
segment_tree() {
}
segment_tree(size_t s) {
n = 1;
while (n < s) n <<= 1;
t.assign(2 * n, 0);
lazy = t;
}
void push(int v, int vl, int vr) {
if (vl == vr || lazy[v] == -1) return;
lazy[v << 1] = lazy[v];
lazy[v << 1 | 1] = lazy[v];
int vm = vl + vr >> 1;
t[v << 1] = lazy[v] * (vm - vl + 1);
t[v << 1 | 1] = lazy[v] * (vr - vm);
lazy[v] = -1;
}
void update(int l, int r, int x, int v, int vl, int vr) {
push(v, vl, vr);
if (l > vr || vl > r) return;
if (l <= vl && vr <= r) {
t[v] = x * (vr - vl + 1);
lazy[v] = x;
return;
}
int vm = vl + vr >> 1;
update(l, r, x, v << 1, vl, vm);
update(l, r, x, v << 1 | 1, vm + 1, vr);
t[v] = t[v << 1] + t[v << 1 | 1];
}
void update(int l, int r, int x) {
update(l, r, x, 1, 0, n - 1);
}
int query(int l, int r, int v, int vl, int vr) {
push(v, vl, vr);
if (l > vr || vl > r) return 0;
if (l <= vl && vr <= r) return t[v];
int vm = vl + vr >> 1;
int q1 = query(l, r, v << 1, vl, vm);
int q2 = query(l, r, v << 1 | 1, vm + 1, vr);
t[v] = t[v << 1] + t[v << 1 | 1];
return q1 + q2;
}
int query(int l, int r) {
return query(l, r, 1, 0, n - 1);
}
private:
int n;
vector<int> t, lazy;
};
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
char x; cin >> x;
arr[i][j] = x - '0';
}
}
if (n > m) {
int brr[m][n];
for (int i = 0; i < m; i++) {
for (int j = 0; j < n; j++) {
brr[i][j] = arr[j][i];
}
}
for (int i = 0; i < m; i++) {
for (int j = 0; j < n; j++) {
arr[i][j] = brr[i][j];
}
}
swap(n, m);
}
vector<segment_tree> st(n);
for (int i = 0; i < n; i++)
st[i] = segment_tree(m);
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
st[i].update(j, j, arr[i][j]);
pref[i][j] = arr[i][j];
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 x = i; x <= i + a - 1; x++) {
st[x].update(j, j + b - 1, Q);
}
}
}
}
int ok = 1;
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
if (st[i].query(j, j) != 0 && st[i].query(j, j) != Q)
ok = 0;
}
}
Q++;
return ok;
};
//O(n*n * 12*m * 12*12)
//O(10000 *120 * 144)
//O(1000000)
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);
}
cout << ans << '\n';
return 0;
}
