#include <stdio.h>
#include <ctype.h>
#define MAX_K 63
#define HLEN ((1<<19)-1)

struct h_p {
  long long i;
  char l[26];
};

struct h_p hash[1<<19];
int k;

long long cas=1;
char now[26];
int last[MAX_K];
int last_len;
int now_len;

int hf() {
  int out=165;
  int i;

  for (i=0;i<26;i++) out=(out+now[i]*991)&HLEN;
  return out;
}

int hf2() {
  int out=179;
  int i;

  for (i=0;i<26;i++) out=(out+now[i]*113)&HLEN;
  return out;
}

int is_in_hash() {
  int h=hf();
  int hadd=-1;
  int ret=0;
  while (hash[h].i==cas) {
    if (!memcmp(hash[h].l,now,26)) {
      ret=1;
      break;
    }
    if (hadd==-1) hadd=hf2();
    h=(h+hadd)&HLEN;
  }
  return ret;
}

void add_to_hash() {
  int h=hf();
  int hadd=-1;
  while (hash[h].i==cas) {
    if (hadd==-1) hadd=hf2();
    h=(h+hadd)&HLEN;
  }
  hash[h].i=cas;
  memcpy(hash[h].l,now,26);
}

int main(void) {
  int i,c;
  int count;
  int res;

  while (scanf("%d",&k),k) {
    while (c=getchar(),c!='\n') {}
    now_len=0;
    last_len=0;
    count=0;
    res=-1;
    for (i=0;i<26;i++) now[i]=1;

    while (c=getchar(),c!='\n') {
      count++;
      if (!isalpha(c)) continue;
      c=tolower(c)-'a';
      
      if (now_len<k) {
	now[c]+=2;
	last[now_len]=c;
	now_len++;
      } else {
	now[last[last_len]]-=2;
	last[(last_len+k)&MAX_K]=c;
	now[c]+=2;
	last_len=(last_len+1)&MAX_K;
      }
      
      if (now_len==k) /* try this*/ {
	if (is_in_hash()) {
	  while (c=getchar(),c!='\n' && c!=EOF) {}
	  res=count-1;
	  break;
	} else {
	  add_to_hash();
	}
      }
    }

    printf("%d\n",(res==-1) ? count : res);
    cas++;
  }

  return 0;
}