#include <bits/stdc++.h>
using namespace std;
int main()
{
int a,b,h;
float s;
cin>>a>>b>>h;
s=(a+b)*h/2.0;
printf("%.1f",s);
	return 0;
}

/**************************************************************
	Problem: 1329
	User: wqc123
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/