#include<iostream>
#include<cstdio>
using namespace std;
int main()
{
	float a,m,k,sc;
	cin>>a>>m>>k;
	if((a/20)>m)
	{
	sc=(a/20-m)*k;
	printf("%.2f",sc);
}
else
printf("%.2f",0.00);
}