#include<iostream> using namespace std; int main(){ int a[100],n,count=-1; cin>>n; for(int i=0;i<n;i++){ cin>>a[i]; } for(int i=0;i<n;i++){ if(i==a[i]){ count=a[i]; break; } } if(count==-1) cout<<"N"; else cout<<count; return 0; }