#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define sz(x) ((int)x.size())
#define MP make_pair
#define ft first
#define sd second
#define pii pair<int, int>
#define PB push_back
using namespace std;
using namespace __gnu_pbds;
template <class T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
const int N = 2510;
const int oo = 2e9;
int n, m, ans, a[N][N], mx = oo, pr[N][N], best[N], nt[N][N], mnw = oo;
int main(){
ios_base::sync_with_stdio(0); cin.tie(0);
cin >> n >> m;
for (int i = 1; i <= n; i++)
for (int j = 1; j <= m; j++){
char c; cin >> c;
a[i][j] = (c == '1' ? 1 : 0);
}
for (int j = 1; j <= m; j++){
for (int i = 1; i <= n; ){
if (a[i][j] == 0)
i++;
else {
int i1 = i;
while (i1 <= n && a[i1][j] == 1)
i1++;
mx = min(mx, i1 - i);
i = i1;
}
}
}
for (int i = 1; i <= n; i++){
for (int j = 1; j <= m; ){
if (a[i][j] == 0)
j++;
else {
int i1 = j;
while (i1 <= m && a[i][i1] == 1)
i1++;
mnw = min(mnw, i1 - j);
j = i1;
}
}
}
fill(best, best + n + 1, oo);
for (int i = 1; i <= n; i++){
int pre = 0;
for (int j = 1; j <= m; j++){
if (a[i][j] == 0)
pre = j;
pr[i][j] = pre + 1;
}
pre = m + 1;
for (int j = m; j > 0; j--){
if (a[i][j] == 0)
pre = j;
nt[i][j] = pre - 1;
}
}
for (int j = 1; j <= m; j++){
int lf = 0, rt = m + 1, len = 0;
for (int i = 1; i <= n; i++)
if (a[i][j] == 1){
lf = max(lf, pr[i][j]);
rt = min(rt, nt[i][j]);
len++;
best[len] = min(best[len], rt - lf + 1);
} else {
len = lf = 0;
rt = m + 1;
}
lf = 0, rt = m + 1, len = 0;
for (int i = n; i > 0; i--)
if (a[i][j] == 1){
lf = max(lf, pr[i][j]);
rt = min(rt, nt[i][j]);
len++;
best[len] = min(best[len], rt - lf + 1);
} else {
len = lf = 0;
rt = m + 1;
}
}
for (int i = 1; i <= mx; i++) {
if (best[i] == oo) return -1;
ans = max(ans, min(mnw, best[i]) * i);
}
cout << ans << '\n';
return 0;
}
