代码如下：

#include<bits/stdc++.h>
using namespace std;
int main() {
	int n, m;
	cin >> n >> m;
	int a[n][m];
	for (int i = 0; i < n; i++) {
		string s;
		cin >> s;
		for (int j = 0; j < m; j++) {
			a[i][j] = s[j] - '0';
		}
	}
	int ans = 0;
	for (int y = 1; y <= m; y++) {
		for (int i = 1; i <= n; i++) {
			for (int j = 0; j + i <= n; j++) {
				for (int k = 0; k + y <= m; k++) {
					int sum_black = 0, sum_white = 0;
					for (int x = 0; x < y; x++) {
						for (int c = j; c < i + j; c++) {
							(a[c][k + x] == 1) ? sum_black++ : sum_white++;
						}
					}
					if (sum_black == sum_white) ans = i * y;
				}
			}
		}
	}
	cout << ans;
	return 0;
}