I. Divisability

#include <iostream>
#include<bits/stdc++.h>
#define ll long long
#define ull unsigned long long
using namespace std;

ll Sum(ll n){
return (n*(n+1))/2 ;
}
ll Sum_of_Devisors(ll a, ll b, ll c){
        ll right = Sum(b / c);
        ll left = Sum((a - 1) / c);
        return (right - left) * c;
    }
int main() {
   ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);

ll a,b,c;
cin>>a>>b>>c;
cout<<(Sum_of_Devisors( a,  b, c));

    return 0;
}