import sys

# sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.buffer.readline())
def LI(): return list(map(int, sys.stdin.buffer.readline().split()))
def LI1(): return list(map(int1, sys.stdin.buffer.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def BI(): return sys.stdin.buffer.readline().rstrip()
def SI(): return sys.stdin.buffer.readline().rstrip().decode()
dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = 10**16
# md = 998244353
md = 10**9+7

from heapq import *

def D(x, y):
    res = 0
    for c0, c1 in zip(x, y): res += c0 != c1
    return res

n, l = LI()
gg = [BI() for _ in range(n)]

ans = 0
hp = []
fin = [False]*n
fin[0] = True
for i in range(1, n):
    heappush(hp, (D(gg[0], gg[i]), i))

cnt = 1
while cnt < n:
    d, i = heappop(hp)
    if fin[i]: continue
    fin[i] = True
    ans += d
    cnt += 1
    for j in range(n):
        if fin[j]: continue
        heappush(hp, (D(gg[i], gg[j]), j))

print(ans)