#include<bits/stdc++.h>
using namespace std;
int main() {
	float a,b,h,t;
	cin>>a>>b>>h;
	t=(a+b)*h/2/1.0;
	printf("%.1f",t);
	
}

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