#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <iostream>
#include <queue>
#include <list>
#include <stack>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef unsigned long long ull;

static const ull B = 100000007ULL;

bool check(int N, const string& str){
  REP(i,1,N){
    ull h1 = (str[i-1]);
    ull h2 = (str[i]);
    ull t = B;
    REP(j,i,N+1){
      if(j+(j+1-i)>N) break;
      //cout << i << " " << j << " " << str.substr(i-1,j-i+1) << endl;

      if(h1 == h2){
        if(str.substr(i-1,j-i+1) == str.substr(j,j-i+1)) return true;
      }

      h1 = h1*B + (str[i-1]);
      if(j+j-i+2>=N) break;
      h2 = ((h2 - str[j]*t)*B + (str[j+j-i+1]) ) * B + (str[j+j-i+2]);
    }
  }
  return false;  
}

int main(){
  int N;
  cin >> N;
  string str;
  cin >> str;

  if(check(N, str)){
    cout << "YES" << endl;
  }else{
    cout << "NO" << endl;
  }
  return 0;
}