N,M=map(int,input().split())
v=[[[] for i in range(N+1)] for j in range(M+1)]
for i in range(N*M):
  n,m=map(int,input().split())
  l=n-4
  if l<=0:
    l=1
  for x in range(l,n+1):
    v[m][x].append(i+1)
  if n==1:
    v[m][N-3].append(i+1)
result=10**9
for i in range(1,M+1):
  for j in range(1,N-2):
    a,b,c,d,e=v[i][j][:]
    count=0
    x=0
    for k in range(5):
      a=v[i][j][k]
      if a<=x:
        continue
      e=(a-x+4-k)//(5-k)
      x+=e*(5-k)
      count+=e
    result=min(result,count-1)
print(result)