if __name__  == '__main__':
    k = int(input())
    str1 = input()
    length = len(str1)
    #print(length)
    if length%k != 0:
        print(-1)
        exit()
    n = length // k
    ans = 0
    
    for i in range(n):
        cnt = [0] * 26
        maxx = 0
        for j in range(k):
            idx = j*n + i
            idx = int(ord(str1[idx])- ord('a'))
            cnt[idx] += 1
        #print(cnt)
        maxx = max(cnt)
        ans += k - maxx
        
    print(ans)
            #print(str1[idx])