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

int main() {
    string s;
    getline(cin, s); 
    int a, b, last;

    s.erase(remove(s.begin(), s.end(), ' '), s.end());

    // Check if input contains "..."
    bool str_end = s.find("...") != string::npos;

    vector<int> terms;
    string temp = "";
    for (char ch : s) {
        if (isdigit(ch)) {
            temp += ch;
        } else if (ch == '+' || ch == '.') {
            if (!temp.empty()) {
                terms.push_back(stoi(temp));
                temp = "";
            }
        }
    }
    if (!temp.empty()) terms.push_back(stoi(temp)); // last number if no str_end

    a = terms[0];
    b = terms[1];

    
    cin >> last;
     
	
    int d = b - a;
    int n = (last - a) / d + 1;
    int sum = (2 * a + (n - 1) * d) * n / 2;

    cout << sum << "\n";
    return 0;
}