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