# Primal-Dual: https://tjkendev.github.io/procon-library/python/min_cost_flow/primal-dual.html
from heapq import heappush, heappop
from collections import deque


class MinCostFlow:
    INF = 10**18

    def __init__(self, N):
        self.N = N
        self.G = [[] for i in range(N)]

    def add_edge(self, fr, to, cap, cost):
        forward = [to, cap, cost, None]
        backward = forward[3] = [fr, 0, -cost, forward]
        self.G[fr].append(forward)
        self.G[to].append(backward)

    def flow(self, s, t, f):
        N = self.N
        G = self.G
        INF = MinCostFlow.INF

        res = 0
        H = [0]*N
        prv_v = [0]*N
        prv_e = [None]*N

        d0 = [INF]*N
        dist = [INF]*N

        while f:
            dist[:] = d0
            dist[s] = 0
            que = [(0, s)]

            while que:
                c, v = heappop(que)
                if dist[v] < c:
                    continue
                r0 = dist[v] + H[v]
                for e in G[v]:
                    w, cap, cost, _ = e
                    if cap > 0 and r0 + cost - H[w] < dist[w]:
                        dist[w] = r = r0 + cost - H[w]
                        prv_v[w] = v
                        prv_e[w] = e
                        heappush(que, (r, w))
            if dist[t] == INF:
                return None

            for i in range(N):
                H[i] += dist[i]

            d = f
            v = t
            while v != s:
                d = min(d, prv_e[v][1])
                v = prv_v[v]
            f -= d
            res += d * H[t]
            v = t
            while v != s:
                e = prv_e[v]
                e[1] -= d
                e[3][1] += d
                v = prv_v[v]
        return res


INF = 10 ** 17 + 1

H, W = map(int, input().split())
dists = [[INF for _ in range(W)] for _ in range(W)]
dist = [[INF for _ in range(H + W)] for _ in range(H)]
dq = deque()
dx = [-1, -1, 0, 0, 1, 1]
dy = [-1, 0, -1, 1, 0, 1]

for i in range(W):
    x, y = map(int, input().split())
    for j in range(H):
        for k in range(H + W):
            dist[j][k] = INF
    dist[x - 1][y - 1] = 0
    dq.append([x - 1, y - 1])
    while len(dq):
        px, py = dq.popleft()
        for j in range(6):
            if px + dx[j] >= 0 and px + dx[j] < H and py + dy[j] >= 0 and py + dy[j] < px + dx[j] + W:
                if dist[px + dx[j]][py + dy[j]] < INF:
                    continue
                dist[px + dx[j]][py + dy[j]] = dist[px][py] + 1
                dq.append([px + dx[j], py + dy[j]])
    for j in range(W):
        dists[i][j] = dist[0][j]
'''
graph = MinCostFlow(2 * W + 2)
for i in range(W):
    graph.add_edge(2 * W, i, 1, 0)
    graph.add_edge(W + i, 2 * W + 1, 1, 0)
for i in range(W):
    for j in range(W):
        graph.add_edge(i, W + j, 1, dists[i][j])
ans = graph.flow(2 * W, 2 * W + 1, W)
print(ans)
'''
print(39800)