rt

#include<bits/stdc++.h>
using namespace std;
int all,each,month;
int check(double a){
	double money=all;
	for(int i=1;i<=month;i++){
	    money=money*(1+a)-each;
	}
	return money;
}
double search(double l, double r)
{
    const double eps = 0.0001;
    double mid;
    while (r - l > eps)
    {
    	
        mid = (l + r) / 2;
        if (check(mid)<0) r = mid;
        else if(check(mid)>0) l = mid;
        else return mid;
    }
    return mid;
}
int main(){
	cin>>all>>each>>month;
	printf("%.1lf",search(0,5));
	return 0;
}