小红的字符串

import sys

while True:
    try:
        n = input()
        line = input()
        n = int(n)
        if n > 62:
            print(-1)
        else:
            space = [-1]*123
            for i in range(48,58):
                space[i] = 0
            for i in range(65,91):
                space[i] = 0

            for i in range(97,123):
                space[i] = 0
            for i in line:
                space[ord(i)] += 1
            res = 0
            for x in range(123):
                while space[x] > 1:
                    i = x
                    while space[i] != 0:
                        if i ==  122:
                            i = 65
                        elif i == 90:
                            i = 48
                        elif i == 57:
                            i = 97
                        else:
                            i += 1
                        res += 1
                    space[i] += 1
                    space[x] -= 1
#             print(space)
            print(res)

        exit(0)
    except:
        break