#include<bits/stdc++.h>
using namespace std;
int main(){
	int q,w,e;
	double z;
	cin>>q>>w>>e;
	printf("%.1f",(q+w)*e/2);
}

/**************************************************************
	Problem: 1329
	User: wuzhijing
	Language: C++
	Result: Wrong Answer
****************************************************************/