int key[128] = {0,};

int right=-1, left=-1, cnt=0, res=0;

int size = str.size();

while(left <= right){

if(cnt<M || (cnt==M && key[str[right+1]]>0)){

right++;

key[str[right]]++;

if(key[str[right]] == 1)

cnt++;

}

else{

left++;

key[str[left]]--;

if(key[str[left]] == 0)

cnt--;

}

res = max(right - left, res);

if(right == size - 1)

break;

}

cout<<res<<"\n";

ios::sync_with_stdio(false);

cin.tie(0);

while(1){

int M;

string str;

cin>>M;

if(M == 0)

break;

getline(cin, str);

getline(cin, str);

find(M, str);

}

return 0;

}