#include<bits/stdc++.h>
using namespace std;
int main() {
	float a,b,h,s;
	cin>>a>>b>>h;
	s=(a+b)*h/2;
	printf("%.1f",s);
    return 0;
}

/**************************************************************
	Problem: 1329
	User: huangcanwu
	Language: C++
	Result: Accepted
	Time:6 ms
	Memory:2072 kb
****************************************************************/