Jolly Jumpers

Corrected & Accepted !!! (Thanks to xanden for his comments) 
____________________________________________________________________
Question: Jolly Jumpers
____________________________________________________________________

#include "iostream.h"

int main()
{
 int n, n1, n2, arr[3001], diff, flag;
 while(cin >> n) {
     for(int i = 1; i < n; i++) arr[i] = 0;
     cin >> n1;
     flag = 0;
     for(int i = 1; i < n; i++) {
         cin >> n2;
         diff = (n1 > n2)?(n1 - n2):(n2 - n1);
         if(diff > 0 && n > diff) {
             if(arr[diff] != 1) arr[diff] = 1;
             else flag = 1;
         }
         else flag = 1;
         n1 = n2;
     }
     if(flag == 0) cout << "Jolly" << endl;
     else cout << "Not jolly" << endl;
 }
 return 0;
}
____________________________________________________________________