#include<bits/stdc++.h>
using namespace std;
int main(){
	int a,b,c;
	float s;
	cin>>a>>b>>c;
	s=(a+b)*c/2.0;
	cout<<s;
	return 0;
}
/**************************************************************
	Problem: 1329
	User: lijinkai
	Language: C++
	Result: Wrong Answer
****************************************************************/