#include<bits/stdc++.h>
using namespace std;
#define ll long long
int main()
{
    string s; cin>>s;
    int n; cin>>n;

    int x = s[0] - '0';
    int y = s[2] - '0';
    int diff = y-x;


    ll ex = x+ (n-1)*diff;


    ll t_sum = (ex*(ex+1))/2;
    ll fi_sum = ((x-1)*(x))/2;
    ll sum = 0;

    for(int i=x+1; i<ex; i+=diff)
        sum+=i;


    cout<<t_sum-fi_sum-sum<<endl;





    return 0;
}