#include <iostream>
#include <vector>
#include <algorithm>
#include <unordered_map>
#include<unordered_set>
#define ll long long
using namespace std;

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