#include<iostream>
#include<map>
#include<queue>

using namespace std;

typedef map<char,int> MCI;
typedef queue<char> QC;

void erase(MCI& m, QC& q, int& akt) {
	if (q.empty()) exit(0);
	char rm=q.front(); q.pop();
	if (m[rm]<=0) exit(0);
	akt--; m[rm]--;
	while (m.count(rm)>0 && m[rm]>0) erase(m,q,akt);
	if (m.count(rm)>0) m.erase(rm);
}

int main() {
	freopen("broken.in","r",stdin);
	unsigned int n; cin >> n;
	while (n) {
		char c=0;
		while (c!='\n') cin.get(c);
		int max=0,akt=0;
		MCI m; QC q;
		for (cin.get(c); c!='\n'; cin.get(c)) {
			if (m.count(c)==0 && m.size()>=n) erase(m,q,akt);
			q.push(c); akt++; m[c]++;
			if (akt>max) max=akt;
		}
		cout << max << endl;
		cin >> n;
	}
}