#include<bits/stdc++.h>
using namespace std;
int main(){
	float S,a,b,h;
	cin>>a>>b>>h;
	S=(a+b)*h/2;
	cout<<S;
	return 0;
}
/**************************************************************
	Problem: 1329
	User: zengyixuan
	Language: C++
	Result: Wrong Answer
****************************************************************/