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

	float a,b,h,c;
	cin>>a>>b>>h;
	c=(a+b)*h/2;
	printf("%.1f",c);


	return 0;
}

/**************************************************************
	Problem: 1329
	User: zhuangsongyu
	Language: C++
	Result: Accepted
	Time:14 ms
	Memory:2072 kb
****************************************************************/