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

/**************************************************************
	Problem: 1329
	User: zhengzihao3
	Language: C++
	Result: Wrong Answer
****************************************************************/