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

} 
/**************************************************************
	Problem: 1329
	User: zhangyilei
	Language: C
	Result: Compile Error
****************************************************************/