#include<bits/stdc++.h>
using namespace std;
int main(){
	int a,b,h;
	cin>>a>>b>>h;
	printf("%.1f",(a+b)*h*1.0/2);
}
/**************************************************************
	Problem: 1329
	User: baizhoudeyueguang
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/