#!/usr/bin/env python3
n,l = [int(i) for i in input().split()]
trie = [('',{})] # string, children

for t in range(n):
    w = input()
    ni,si = 0,0         # node index, string index
    for wi in range(l): # word index
        s, nxt = trie[ni]
        if si < len(s):
            if w[wi] != s[si]:
                # split node
                trie[ni] = (s[:si],{s[si] : len(trie), w[wi] : len(trie)+1})
                trie += [(s[si+1:], nxt), (w[wi+1:], {})]
                break
            else:
                # follow node
                si += 1
        else:
            if w[wi] not in nxt:
                # new child
                nxt[w[wi]] = len(trie)
                trie.append((w[wi+1:],{}))
                break
            else:
                # next child
                ni = nxt[w[wi]]
                si = 0

print(n*l - sum([len(s) for s,nxt in trie if len(nxt) == 0]))