h, w = map(int, input().split())
A = [input() for _ in range(h)]
A.sort()

dp = [1] * h

for i in range(1, h):
    for j in range(i):
        if all(A[j][k] == "0" or A[i][k] == "1" for k in range(w)):
            dp[i] = max(dp[i], dp[j] + 1)

print(h - max(dp))