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

#define ll long long
#define endl '\n'
#define print(v) for(auto e:v) cout<<e<<" "; cout<<endl;

void solve() {
    string s ; cin >>s;
    int n; cin >> n ;
    int a = s[0] -'0';
    int b = s[2]-'0';
    ll dif =  b - a;
    ll ki = (n - 1) * dif;
    ki +=  (2*a);
    ki *= n;
    ki /= 2;
    cout << ki << endl;}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int t = 1;// cin >> t;
    while(t--){
        solve();
    }
    return 0;
}