思路：

直接二分平均值，然后把数组全部减去平均值，贪心求每一列的最大值，然后判断是否合法就行了

$code$

#include<iostream>
#include<cstdio> 

using namespace std;

int n, m, tot;
double a[100010];

bool check(double x) {
    
	double s1 = 0, s2 = 0, s3 = 0;
	tot = 0;
	for(int i = 1; i <= n; i ++) {
    
		s1 = s2 = a[++ tot] - x;
		for(int j = 2; j <= m; j ++)
			s1 = s1 + a[++ tot] - x, s2 = max(s1, s2);
		s3 += s2;
	}
	return s3 >= 0;
}

int main() {
    
	scanf("%d%d", &n, &m);
	for(int i = 1; i <= n; i ++)
		for(int j = 1; j <= m; j ++)
			tot ++, scanf("%lf", &a[tot]);
	double l = 0, r = 1000000000.0, ans = 0;
	while(l + 0.000001 < r) {
    
		double mid = (l + r) / 2;
		if(check(mid)) l = mid, ans = max(ans, mid);
		else r = mid;
	}
	printf("%.4lf", ans);
	return 0;
	
}