思路：

直接推式子，然后枚举一行多少个箱子，然后判断就行

$code$

#include<iostream>
#include<cstdio>

using namespace std;

long long l, n, m;

int main() {
    
	scanf("%lld%lld%lld", &l, &n, &m);
	if(n % l == 0 && m % l == 0) {
    
		printf("0.00000");
		return 0;
	}
	register long long ans = 0;
	register long double k = 0.0;
	for(register long long i = 1; i <= n; ++ i) {
    
		long long r = i - 1;
		if(n - r * l > 0) {
    
			register long double x = (n * 1.0 - r * l * 1.0) / (i * 1.0);
			if(x < 0) continue;
			register long long h = (m * 1.0 - x) / (l * 1.0 + x);
			if(h < 0) continue;
			if(m - l * h >= 0) {
    
				if((m - l * 1.0 * h) / (h + 1.0) == x) {
    
					if(h * r > ans) ans = h * r, k = x;
				}
			}
		}
		else break;
	}
	if(ans == 0) printf("-1");
	else printf("%.5Lf", k);
	return 0;
}