#include<iostream>

using namespace std;

int main(void)
{

	float s;
	int a,b,h;
	cin>>a>>b>>h;
	
	s=(a+b)*h/2.0;
	
	printf("%.1f", s);
	return 0;
}
/**************************************************************
	Problem: 1329
	User: 231308061
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/