#include <bits/stdc++.h>
using namespace std;
#define fast ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define ll long long
int main()
{
    fast
    /*ll t;
    cin>>t;
    while(t--) solve*/

    string s;
    long long n;
    cin>>s>>n;

    vector <long long> num;
    num.push_back(s[0]-'0');
    num.push_back(s[2]-'0');

    long long d = num[1]-num[0];
    long long f = num[0], sum=0;

    for (long long i=0; i<n; i++){
        sum+=f;
        f+=d;
    }

    cout<<sum<<endl;
    return 0;
}