#include<bits/stdc++.h>
using namespace std;
int main(){

	float a,b,h;
	cin>>a>>b>>h;
	cout<<fixed<<setprecision(1)<<(a+b)*h/2<<endl;


	return 0;
}

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