15
MIRZOYANOVECLOX
 23
AABBBAAACCCCCAAADDDDDDD
 2
QA
 11
GGRRAATTZZZ
 6
AZAZAZ
 10
SQSQSQSQTG
 3
KEK
 100
URXCAIZFIBNJTPCZHBQIBCILLPXZCFGMKKZMNPLCYGAVJVIBMCZEBSJWPSCPQDYCTTKPOKIJRSKIZPDGCHVOUTMPNECYORSFZFNC
 3
LOL
 2
WW
 8
PUTINVOR
 26
QWERTYUIOPASDFGHJKLZXCVBNM
 5
AZAZA
 9
EGORLETOV
 7
KADUROV
 9
MIKEPIDOR
 9
AAAAAAAAA
 100
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
 5
FUFEL
 50
NYQAHBYYOXLTRYQDMVENEMAQNBAKGLGQOLXNAIFNQTOCLNNQIA

MI
 DD
 QA
 ZZ
 AZ
 SQ
 KE
 IB
 LO
 WW
 PU
 QW
 AZ
 EG
 KA
 MI
 AA
 AA
 FU
 YQ

// http://codeforces.com/contest/977/problem/B

var arg1 = readline();
var arg2 = readline();

var length = parseInt(arg1);
var string = arg2;

function main(length, string) {
    var hash = {};

    for (var i = 0; i < length - 1; i++) {
        var substring = `${string[i]}${string[i + 1]}`;
        if (hash[substring]) {
            hash[substring] = hash[substring] + 1;
        } else {
            hash[substring] = 1;
        }
    }

    var twogrammaList = Object.keys(hash);
    var biggest = twogrammaList[0];

    for (var j = 1; j < twogrammaList.length; j++) {
        var tg = twogrammaList[j];

        if (hash[tg] > hash[biggest]) {
            biggest = twogrammaList[j];
        }
    }

    return biggest;
}

print(main(length, string));
