Maths

[Kalcor]

Here is a question link:
http://codeforces.com/problemset/problem/239/A

Here is a code:

#include <bits/stdc++.h>


#define fl(i, n) for(int i = 0; i < n; i++)

#define ll   long long
#define nl   endl
#define pb   push_back
#define mp   make_pair
#define PII  pair<int,int>

#define EPS  1e-9
#define INF  1e9


using namespace std;
int main()
{
    int y, k, n;
    scanf("%d%d%d", &y, &k, &n);
    int x = k - y % k;
    int top = n - y;
    if (x <= top)
    {
        printf("%d", x);
        x += k;
        while (x <= top)
        {
            printf(" %d", x);
            x += k;
        }
        printf("\n");
    }
    else
    {
        printf("-1\n");
    }
    return 0;
}

I don't get this part:

int x = k - y % k; // line 21 
    int top = n - y; // line 22
//and
x += k;// line 26