#include<bits/stdc++.h>
 
using namespace std;
 
class Solution {
public:
    int topKFrequent(string s,int k) {
        int left,right,cnt,ans;
        char prev;
        left=ans=0;
        right=1;
        unordered_map<char,bool>m;
        //prev=s[0];
        m[s[0]]=true;
        while(right<s.size()) {
           if(!m[s[right]]) {
                m[s[right]]=true;
           }
           if(m.size()>k) {
                //cout<<right<<endl;
                ans=max(ans,right-left);
                left=right;
                m.erase(m.begin(),m.end());
                m[s[left]]=true;
           }
           //prev=s[right];
           right++;
        }
        ans=max(ans,right-left);
        return ans;
    }
};
int main()
{
    string s;
    int k,ans;
    cin>>k;
    cin>>s;
    Solution sol;
    ans=sol.topKFrequent(s,k);
    cout<<ans<<endl;
}