#include <bits/stdc++.h>
using namespace std;
int main(){
    string s;
    getline(cin, s);
    int a[100], count = 0;
    string temp = "";
    for(int i = 0; i < s.length(); i++){
        char ch = s[i];
        if(isdigit(ch)){
            temp = temp + ch;
        }
        else{
            if(temp != ""){
                a[count] = stoi(temp);
                count++;
                temp = "";
            }
        }
        if(count == 2){
            break;
        }
    }
    long long sum = 0, diff = a[1] - a[0], n;
    cin >> n;
    for(int i = 2; i < n; i++){
       a[i] = a[i-1] + diff;
    }
    for(int i = 0; i < n; i++){
       sum += a[i];
    }
    cout << sum << endl;
    return 0;
}