#!/usr/bin/env PyPy3

from collections import Counter, defaultdict, deque
import itertools
import re
import math
from functools import reduce
import operator
import bisect
import heapq
import functools
mod=10**9+7

import sys
input=sys.stdin.readline

n,m=map(int,input().split())
s=[input().rstrip() for _ in range(n)]
ans = 0
for i in range(1,n):
    now = 0
    for k in range(m):
        if now < m and s[0][k] == s[i][now]:
            now += 1
    if now != m - 1:
        print(0)
        exit()
if n == 2:
    c = 0
    for j in range(m):
        if s[0][j] == s[1][j]:
            c += 1
    if c >= m - 2:
        print(2)
    else:
        print(1)
else:
    l = [0] * 26
    for i in range(n):
        tmp = [0] * 26
        for j in range(m):
            tmp[ord(s[i][j])-ord('a')] += 1
        for j in range(26):
            l[j] = max(l[j],tmp[j])
    if sum(l[j]) == m + 1:
        print(1)
    else:
        print(0)