n,m=map(int,input().split())
c=[[0]*(n+1) for i in range(m)]
for i in range(n*m):
	s,t=map(int,input().split())
	s-=1
	t-=1
	c[t][s]=max(0,i-4)
	if s==0:
		c[t][n]=max(0,i-4)
a=n*m
for i in range(m):
	for j in range(n-3):
		g=c[i][j:j+5]
		g.sort()
		b=0
		bb=0
		for k in range(5):
			if g[k]<=bb:
				continue
			b+=(g[k]-bb+5-k-1)//(5-k)
			bb+=b*(5-k)
		a=min(a,b)
print(a)