#include <bits/stdc++.h>
#define nl '\n'
#define ll long long
#define all(c) c.begin(),c.end()
#define print(c) for(auto e : c) cout << e << " "; cout << nl
using namespace std;

int main()
{
    ios_base::sync_with_stdio(false); cin.tie(NULL);

    string s; cin >> s;
    int n; cin >> n;

    int firstNum = s[0] - '0';
    int d = (s[2] - '0') - (s[0] - '0');

    cerr << firstNum << " " << d << nl;

    ll sum = (n * (2 * firstNum + (n - 1) * d)) / 2;

    cout << sum << nl;

    return 0;
}