n, m = map(int, input().split())
a = [[0] * (n + 1) for _ in range(m)]
for i in range(n * m):
	s, t = map(int, input().split())
	s -= 1; t -= 1
	a[t][s] = max(i - 4, 0)
	if s == 0:
		a[t][n] = max(i - 4, 0)
ans = n * m
for i in range(m):
	for j in range(n - 3):
		b = sorted(a[i][j:j + 5])
		t = x = 0
		for k, v in enumerate(b):
			if v <= x:
				continue
			t += (v - x + 4 - k) // (5 - k)
			x += (v - x + 4 - k) // (5 - k) * (5 - k)
		ans = min(ans, t)
print(ans)