#include<bits/stdc++.h>
using namespace std;

#define ll long long
#define irep(i, l, r) for(ll (i) = (l); (i) <= (r); ++ (i))
#define drep(i, r, l) for(ll (i) = (r); (i) >= (l); -- (i))


inline ll read(){
    ll s = 0, w = 1;
    char ch = getchar();
    while(ch > '9' || ch < '0'){
        if(ch == '-')w = - 1;
        ch = getchar();
    }
    while(ch >= '0' && ch <= '9'){
        s *= 10, s += (ch ^ 48);
        ch = getchar();
    }
    return s * w;
}
const int N = 500000;
const int mod = 1000000000 + 7;

void solve(){
    int n = read();
    string s;
    cin >> s;
    vector<int>usd(26);
    vector<char>fun(26);
    vector<string>vec(n);
    irep(i, 0, n - 1){
        char cur = 'a';
        irep(ii, 0, 25)usd[ii] = 0;
        drep(j, i, 0){
            if(usd[s[j] - 'a']){
                continue;
            }
            else{
                usd[s[j] - 'a'] = 1;
                fun[s[j] - 'a'] = cur;
                ++ cur;
            }
        }
        irep(ii, 0, i)vec[i] = vec[i] + fun[s[ii] - 'a'];
    }
    sort(vec.begin(), vec.end());
    cout << vec.back();
}

int main(){
    int T = 1;
    solve();
}