void solve()
{
    ll a, b, c;
    cin >> a >> b >> c;
    if (a > b)
        swap(a, b);
    // sum from 1 to n without adding the number which are divisible by c
    ll whole_sum = (((b * (b + 1)) / 2) - ((a * (a - 1)) / 2)) - (((((b / c) * ((b / c + 1))) / 2) - ((((a - 1) / c) * (((a - 1) / c) + 1)) / 2)) * c);
    // the sum of the numbers which are divisible by c after dividing it by c
    ll adding_sum = (((b / c) * ((b / c + 1))) / 2) - ((((a - 1) / c) * (((a - 1) / c) + 1)) / 2);
    ll final_sum = whole_sum + adding_sum;
    cout << final_sum << endl;
}