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

int ctoi(char &c){
    switch(c){
        case '1': return 1;
        case '2': return 2;
        case '3': return 3;
        case '4': return 4;
        case '5': return 5;
        case '6': return 6;
        case '7': return 7;
        case '8': return 8;
        case '9': return 9;
        default: return 0;
    }
}

int main() {
    double u, b, h;
    cin >> u;
    cin.ignore();
    cin >> b;
    cin.ignore();
    cin >> h;


    printf("%.2f", (u + b) * h / 2);
}