#include <bits/stdc++.h>
using namespace std;
float a,b,h,s;
int main(){
     cin>>a>>b>>h;
     s=(a+b)*h/2;
     printf("%.1f",s);
}
/**************************************************************
	Problem: 1329
	User: chenyichuen
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/