#include<bits/stdc++.h>
using namespace std;
int main(){
	float s,a,b,h;
	cin>>a>>b>>h;
	s=(a+b)*h/2;
	printf("%.1lf",s);
}

/**************************************************************
	Problem: 1329
	User: huxuanchen
	Language: C++
	Result: Accepted
	Time:7 ms
	Memory:2072 kb
****************************************************************/