样例2.9，但是输出是500.0，

#include <bits/stdc++.h>

using namespace std;
double x, y, m;
double check(int lv) {
	double sum=x;
	for (int i=1; i<=m; i++) {
		sum=sum*(1+lv)-y;
	}
	return sum;
}
int main() {
	cin >> x >> y >> m;
	double l=0, r=5;
	while ((r-l)<1e-5) {
		double mid=(l+r)/2;
		if (check(mid)>0) {
			r=mid;
		}
		else {
			l=mid;
		}
	}
	cout << fixed << setprecision(1) << r*100;
	return 0;
}

有毋大佬挑下错？？？