#include <iostream>
using namespace std;

int main() {
    long long l, r, k;
    cin >> l >> r >> k;

    long long first = ((l + k - 1) / k) * k;  
    long long last = (r / k) * k;             

    if (first > last) {
        cout << 0 << endl;
    } else {
        long long count = (last - first) / k + 1;
        long long sum = (first + last) * count / 2;
        cout << sum << endl;
    }

    return 0;
}