#include <iostream>
#include <iomanip>

int main() {
    int a, b, h;
    std::cin >> a >> b >> h;
    double area = static_cast<double>(a + b) * h / 2;
    std::cout << std::fixed << std::setprecision(1) << area << std::endl;
    return 0;
}    
/**************************************************************
	Problem: 1329
	User: linmiaoling
	Language: C++
	Result: Accepted
	Time:6 ms
	Memory:2072 kb
****************************************************************/