#!/usr/bin/env python3

n, l = map(int, input().split())
s = [input() for _ in range(n)]
s.sort()
s = ["_"] + s + ["_"]


def len(a, b):
    i = 0
    while a[i] == b[i]:
        i += 1
    return i


a = 0
for i in range(1, n + 1):
    a += max(len(s[i - 1], s[i]), len(s[i], s[i + 1])) + 1
print(a)