#include<bits/stdc++.h>
using namespace std;
int main(){
	int a,b;
	float h,p=0;
	cin>>a>>b>>h;
	p=(a+b)*h/2;
	printf("%.1f",p);
} 
/**************************************************************
	Problem: 1329
	User: xiaojingxuan
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/