#include<iostream>
#include<iomanip>
using namespace std;
int main(){
	double a,b,h;
	cin>>a>>b>>h;
	double area=(a+b)*h/2;
	cout<<fixed<<setprecision(1)<<area<<endl;
	return 0;
}
/**************************************************************
	Problem: 1329
	User: linyuhang
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/