/// Md Abdur Rahman Salman ///

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

#define endl "\n"
#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;
    int n; cin >> n;
    int a = s[0] - '0'; 
    int y = s[2] - '0';
    int d = y - a;
    long long sum = 1LL * n * (2 * a + (n - 1) * d) / 2;
    cout << sum << endl;
    return 0;
}