#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <iomanip>
#include <algorithm>
#include <utility>
#include <string>
using namespace std;

int main() {
	int a,b,c;
	cin>>a>>b>>c;
	cout<<((a+b)*c)/2;
	return 0;
}