/// Md Abdur Rahman Salman ///

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

#define endl "\n"
#define ll long long
#define bug(x) cout << #x << ": " << x << endl
int32_t main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    string s; cin >> s;
    for(auto c : s)
    {
        if (c == '+' || c == '.') c = ' ';
    }
    stringstream ss(s);
    ll a, y;
    ss >> a >> y;
    ll n; cin >> n;
    if(n == 1)
    {
        cout << a << endl;
        return 0;
    }
    ll d = y - a;
    ll sum = (1LL * n * (2 * a + (n - 1) * d)) / 2;
    cout << sum << endl;
    return 0;
}