n,k = map(int,input().split())
a = sorted(input() for _ in range(n))
x = []
for s in a[::-1]:
    y = []
    for t in x:
        f = g = False
        for i in range(k):
            if s[i]>t[i]: f = True
            if s[i]<t[i]: g = True
        if g: y.append(t)
        if not f: break
    else: y.append(s); x = y[:]
print(n-len(x))