#include <bits/stdc++.h>
#include <vector>
#include <iostream>
#include <map>
#include <string>
#include <cmath>
#include <algorithm>
#include <queue>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repe(i,x,n) for(int i=x; i<(n); ++i)
using ll = long long;
using P = pair<int, int>;
#define pai 3.1415926535897932384
#define INF 100100100
#define _GLIBCXX_DEBUG
#define All(a) (a).begin(),(a).end()
#define rAll(a) (a).rbegin(),(a).rend()

int main() {
    
    double A,B,C;
    char D,E;
    cin >> A >> D >> B >> E >> C;
    cout << fixed << setprecision(2);
    cout << ((A+B)*C)/2 << endl;
 
}