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

int main() {
    string s;
    getline(cin, s); // read the full line like "1+3+..."
    int a, b;

    // Find positions of '+' signs
    size_t plus1 = s.find('+');
    size_t plus2 = s.find('+', plus1 + 1);

    if (plus1 == string::npos || plus2 == string::npos) {
        cerr << "Invalid input format\n";
        return 1;
    }

    // Extract a and b
    a = stoi(s.substr(0, plus1));
    b = stoi(s.substr(plus1 + 1, plus2 - plus1 - 1));

    // Read the last term
    int last;
    cin >> last;

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

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