#include <bits/stdc++.h>
using namespace std;
int main(){
	double a,b,h,S;
	cin>>a>>b>>h;
	S = (a + b) * h / 2;
	printf("%.1f",S);
	return 0;
}

/**************************************************************
	Problem: 1329
	User: zhouhongyi
	Language: C++
	Result: Accepted
	Time:7 ms
	Memory:2072 kb
****************************************************************/