#include<bits/stdc++.h>
using namespace std;
int main(){
	float a,b,h,j;
	cin>>a>>b>>h;
	j=(a+b)*h/2.0;
	printf("%.1f",j);
    
}
/**************************************************************
	Problem: 1329
	User: wangyiyang
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/