reg A fast

#include <iostream>
#include <cmath>
#include <vector>
#include <queue>
#include <algorithm>
#include <string>
#include <stack>
#include <set>
#include <map>
#define pii pair <int, int>
#define pb(x) push_back(x)
using namespace std;
using ll = long long;
using ld = long double;
using db = double;
void cv(vector <int> &v) {
    for (auto x : v) cout << x << ' ';
    cout << "\n";
}

void cvl(vector <ll> &v) {
    for (auto x : v) cout << x << ' ';
    cout << "\n";
}


void cvv(vector <vector <int> > &v) {
    for (auto x : v) cv(x);
    cout << "\n";
}

void cvb(vector <bool> v) {
    for (bool x : v) cout << x << ' ';
    cout << "\n";
}

void cvs(vector <string>  v) {
    for (auto a : v) {
        cout << a << "\n";
    }
}

void cvp(vector <pii> a) {
    for (auto p : a) {
        cout << p.first << ' ' << p.second << "\n";
    }
    cout << "\n";
}

#include <ctime>

bool sh = 0;

ll l, r, a, n;

ll check() {
    ll ans = 0;
    for (int i = 1; i <= n; ++i) {
        for (int j = i + 1; j <= n; ++j) {
            if (j > i && (j - i) % a == 0) {
                ans++;
            }
        }
    }
    return ans;
}

/*
WA
n a = 6 2
my his = 5 6
*/

bool random = 1;

int main() {
    /*ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);*/
    ll l, r, n, a;
    cin >> l >> r >> a;
    n = r - l + 1;
    n = n - a;
    ll N = n / a;
    ll S =  a * (N + 1) * N / 2;
    ll d = n % a * (N + 1);
    //cout << "N = " << N << "\n";
    ll ans = S + d;
    cout << ans;
}