#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define endl "\n"

int main() {
    string s; cin >> s;
    ll n; cin >> n;
    ll a = s[0] - '0';
    ll b = s[2] - '0';
   // cout << a << " " << b << " " << n << endl;

    ll d = b - a;

    ll ans = (n *( 2 * a + (n - 1) * d)) / 2;

    cout << ans << endl;

}