#include <map>
#include <set>
#include <list>
#include <cmath>
#include <ctime>
#include <deque>
#include <queue>
#include <stack>
#include <string>
#include <bitset>
#include <cstdio>
#include <limits>
#include <vector>
#include <climits>
#include <cstring>
#include <cstdlib>
#include <fstream>
#include <numeric>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <unordered_map>

using namespace std;

const int MaxN = 1024;

bool hasRightFormat(string s) {
    if (s.size() < 2) return false;
    for (int i = 2; i < s.size(); i += 2) {
        if (s[i] != s[0]) return false;
    }
    for (int i = 3; i < s.size(); i += 2) {
        if (s[i] != s[1]) return false;
    }
    return true;
}

int lengthByKeepingTwoChars(string s, char a, char b) {
    int N = s.size();
    string tempStr = "";
    for (int i = 0; i < N; ++i) {
        if (s[i] != a && s[i] != b) continue;
        tempStr += s[i];
    }
    if (hasRightFormat(tempStr)) {
        return tempStr.size();
    }
    return 0;
}
int main(){
    int len;
    cin >> len;
    string s;
    cin >> s;
    int sol = 0;
    for (char a = 'a'; a <= 'z'; ++a) {
        for (char b = 'a'; b <= 'z'; ++b) {
            if (a == b) continue;
            int tempSol = lengthByKeepingTwoChars(s, a, b);
            if (tempSol > sol) {
                sol = tempSol;
            }
        }
    }
    cout << sol << "\n";

    return 0;
}