#include<fstream>
using namespace std;
ifstream fin("date.in");
ofstream fout("date.out");

int n,t[100],g[100];
void citire()
{ fin>>n;
  for(int i=1;i<=n;i++)
    {
        fin>>t[i];
        if(t[i])
        {
            g[i]++;
            g[t[i]]++;
        }
    }
}

int main()
{
    citire();
    int ok=1;
    for(int i=1;i<=n;i++)
        if(g[i]>2) ok=0;
    if(ok) fout<<"da";
    else fout<<"nu";
    return 0;
}