#include<bits/stdc++.h>
#include <climits>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using ui = unsigned int;
const ll INF = 1e17+7;
/*
//      _   __  ___  __   __ _______ _______
//     / | / / / _ \ \ \__\ \\__  __\\  __  \ 
//    /  |/ / / /_\ \ \  ___ \  \ \   \ \ \  \ 
//   / /|  / / ____  \ \ \  \ \ _\ \___\ \/  /
//  /_/ |_/ /_/    \__\ \_\  \_\\______\\___/

ID: mdnahidibnaharun
LANG: C++
TASK: practice 
*/

void solve(){
string s; cin >> s;
    ll a=s[0]-'0',b=s[2]-'0';
    ll x; cin >> x;
    ll ans =2*a+(x-1)*(b-a);
  ans *= x;
    cout<<ans/2;
}
int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    // int t;cin>>t;
    // while(t--){
        solve();
    //     cout<<endl;
    // }
return 0;
}