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