N, M = map(int, input().split())
NM = [list(map(int, input().split())) for _ in range(N*M)]
dp = [[0 for _ in range(N+1)] for _ in range(M)]
for i in range(5):
    n, m = NM[i]
    n-=1
    m-=1
    dp[m][n] = 1
    if n==0:
        dp[m][N] = 1
cnt = 1
for i in range(5, N*M):
    n, m = NM[i]
    n-=1
    m-=1
    dp[m][n] = cnt+1
    if n==0:
        dp[m][N] = cnt+1
    cnt+=1
#print(dp)
INF = N*M
ans = INF
for i in range(M):
    for j in range(N):
        S = []
        flag = True
        for k in range(j, j+5):
            if 0<=k<=N:
                S.append(dp[i][k])
            else:
                flag = False
                break
        if flag:
            S.sort()
            tmp = 0
            st = 1
            a = S.count(st)
            while S[-1]>st:
                tmp+=1
                st+=(5-a)
                for idx in range(a, 5):
                    if S[idx]>st:
                        a = idx-1
                        break
            #print(S, tmp)
            ans = min(ans, tmp)
print(ans)