#include<bits/stdc++.h>
using namespace std;
int main(){
	int a,b,c;
	float s;
	cin>>a>>b>>c;
	s=(a+b)*c/2;
	printf("%",s);
	cout<<fixed<<setprecision(1)<<s;
} 
/**************************************************************
	Problem: 1329
	User: huxuanchen
	Language: C++
	Result: Wrong Answer
****************************************************************/