#include <bits/stdc++.h>
#define int long long
using namespace std;
/*
        OBSERVATIONS:


*/
signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    string s;
    cin >> s;
    vector<int> arr;
    for (char ch : s)
    {
        if (isdigit(ch))
        {
            arr.push_back(ch - '0');
        }
    }
    int n;
    cin >> n;
    int res = 2 * arr[0];
    res += (n - 1) * (arr[1] - arr[0]);
    res *= n;
    res /= 2;
    cout << res << endl;
    return 0;
}