#include<bits/stdc++.h>
using namespace std;
int main()
{
	double a,b,c,s;
	cin>>a>>b>>c;
	s=a*c+b*(a+c)+(3.1415926535*c*c)/4;
	s=s/10000;
	cout<<fixed<<setprecision(3)<<s;
	return 0;
}