#include <bits/stdc++.h>
using namespace std;

double w0,w;
int day;

bool check(double mid)	//mid为利率算天数
{
	double t=w0;
	int dy=0;
	while(t>0)
	{
		t=t+mid/100.0*t;
		t-=w;
		dy++;	
	}
	return 	dy>=day;
} 

int main()
{
	cin>>w0>>w>>day;
	double l=1,r=3000;
	while(r-l>1e-8)
	{
		double mid=(l+r)/2;
		if(check(mid))	r=mid;
		else	l=mid;
	}	
	cout<<fixed<<setprecision(1)<<l<<endl;
	
	return 0;
}