#include<bits/stdc++.h>
using namespace std;
int main(){
	float s;
	int a,h,b;
	scanf("%d %d %d",&a,&b,&h);
	s=(a+b)*h/2.0;
	printf("%.1f",s);
}
/**************************************************************
	Problem: 1329
	User: toughuatao1
	Language: C++
	Result: Accepted
	Time:6 ms
	Memory:2076 kb
****************************************************************/