#include<bits/stdc++.h>
using namespace std; 
int main(){
	int a,b,h,s;
	cin>>a>>b>>h;
	s=(a+b)*h/(2+0.0);
	cout<<s;
} 
/**************************************************************
	Problem: 1329
	User: chenjiahui
	Language: C++
	Result: Wrong Answer
****************************************************************/