import sys

def diff_pos(str1, str2):
    for i in range(l):
        if str1[i] != str2[i]:
            return i+1
    return -1


if __name__ == "__main__":
    n, l = [int(x) for x in sys.stdin.readline().split()]
    strings = []
    for _ in range(n):
        strings.append(sys.stdin.readline().strip())
    sorted_strings = sorted(strings)

    total_len = 0  

    for i in range(n):
        unique_len = 0
        cur_string = sorted_strings[i]
        if i > 0:
            prev_string = sorted_strings[i-1]
            unique_len = max(unique_len, diff_pos(cur_string, prev_string))
        if i < n-1:
            next_string = sorted_strings[i+1]
            unique_len = max(unique_len, diff_pos(cur_string, next_string))
        total_len += unique_len
    print(total_len)