#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<(int)(n);i++)
template<class T> inline bool chmax(T&a,T b){if(a<b){a=b;return 1;}return 0;}
template<class T> inline bool chmin(T&a,T b){if(a>b){a=b;return 1;}return 0;}
using ll = long long;

int u,b,h;
char c; 

int main () {
    cin.tie(0); ios::sync_with_stdio(false); 

    cin >> u >> c >> b >> c >> h;
    cout << fixed << setprecision(2) << (u+b)*h/2.0 << "\n";
}