#include<iostream>
#include<iomanip>
using namespace std;
double w0 , w ;
int m;
double l , r = 5, mid;
double judge()
{
	double wi = w0;
	for(int i=1;i<=m;i++)
	{
		wi = wi * (1 + mid) - w;
	}
	return wi ;
}
int main()
{
	cin >> w0 >> w >> m;
	while(r-l>=0.001)
	{
		mid = (l + r) / 2;
		if(judge()>0)
		{
			r = mid;
		}
		if(judge()<0)
		{
			l = mid;
		}
		if(judge()==0)
		{
		break;
	    }
	}
		cout << fixed << setprecision(1) << mid*100;
		return 0;
}