#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(d,N-1){
    int i = d;
    int j = d+1;

    ull h1 = (str[i]);
    ull h2 = (str[j]);
    ull t = B;
    
    while(true){
      //cout << str.substr(i,d-i+1) << " " << h1 << "    " << str.substr(j,d-i+1) << " " << h2 << endl;
      if(h1 == h2) return true;
      i--;
      j++;
      if(i<0 || j>=N) break;
      h1 = h1*B + (str[i]);
      h2 = h2 + (str[j])*t;
      t*=B;
    }
  }
  return false;  
}

int main(){
  ios::sync_with_stdio(false);
  
  int N;
  cin >> N;
  string str;
  cin >> str;

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