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

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

    float d = b - a;

    ll ans = (n / 2.0) * 1ll* ( 2 * a + (n - 1) * d);

    cout << ans << endl;

}