#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <ctime>
#include <cstring>
#include <string>
#include <algorithm>
#include <functional>
using namespace std;

int a, b, h;

int main() {
	cin >> a >> b >> h;
	cout << (a + b) * h * 0.5;
	return 0;
}