#include <bits/stdc++.h> using namespace std;

int main() { double a, b, h; scanf("%lf %lf %lf", &a, &b, &h); printf("%.1lf", (a + b) / 2 * h); return 0; }

#include using namespace std;

int main() { int m, n, s; cin >> m >> n >> s; cout << m * (s / n); return 0; }