#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
ll k, r, l, ans, p, m, temp;
ll d[200010][26];
string s;
ll check(ll a, ll j, ll i){
    /*if (j < 0)
        return d[i][a];
    else*/
        return d[i][a] - d[j][a];
}

int main()
{
    cin >> k >> s;
    if (k == 0){
        temp = 1;
        for (int i = 1; i < s.size(); i++)
            if (s[i - 1] == s[i])
                temp++;
            else{
                ans = max(temp, ans);
                temp = 1;
            }
        ans = max(temp, ans);
        cout << ans << endl;
        return 0;
    }
    d[0][s[0] - 'a']++;
    for (int i = 1; i < s.size(); i++){
        for (int j = 0; j < 26; j++)
            d[i][j] = d[i - 1][j];
        d[i][s[i] - 'a']++;
    }
    /*for (int i = 0; i < 26; i++){
        for (int j = 0; j < s.size(); j++)
            cout << (char) (i + 'a') << " = " << d[j][i] << " ";
        cout << endl;
    }*/
    bool bol = 1;
    for (int i = 0 ; i < 26; i++)
        if (d[s.size() - 1][i] > 1)
            bol = 0;
    if (bol){
        cout << min((ll)s.size(), k + 1) << endl;
        return 0;
    }
    for (int i = 0; i < 26; i++){
        for (int j = 1; j <= s.size(); j++){
            l = j - 1;
            r = s.size();
            while (r - l > 1){
                m = (l + r) / 2;
                if (m - j + 1 - check(i, j - 1, m) <= k)
                    l = m;
                else
                    r = m;
                //cout << l << " " << r << endl;
            }
            ans = max(ans, l - j + 1);
        }
    }
    cout << ans + 1 << endl;
    return 0;
}