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

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