#include <bits/stdc++.h>
using namespace std;
int a,b,h;
double s;
int main(){
    cin>>a>>b>>h;
    s=(a+b)*h/2*1.0;
    printf("%.1f",s);
	return 0;
}
/**************************************************************
	Problem: 1329
	User: zengdongxin
	Language: C++
	Result: Wrong Answer
****************************************************************/