import heapq
import sys
input = sys.stdin.buffer.readline
sys.setrecursionlimit(10 ** 7)
inf = 10 ** 18

N, M = map(int, input().split())
AB = list(list(map(int, input().split())) for _ in range(M))

exist = {1, N}
for a, b in AB:
    exist.add(a)
    exist.add(b)

exist = sorted(exist)
etoi = {e: i for i, e in enumerate(exist)}
size = len(exist)

G = [[] for _ in range(size)]
for i in range(size - 1):
    d = (exist[i+1] - exist[i]) * 2
    G[i].append((i + 1, d))
    G[i + 1].append((i, d))
for a, b in AB:
    d = b + b - a - a - 1
    a = etoi[a]
    b = etoi[b]
    G[a].append((b, d))
    G[b].append((a, d))


dist = [inf] * size
dist[0] = 0
que = [(0, 0)]
while que:
    ds, s = heapq.heappop(que)
    if dist[s] < ds:
        continue
    for t, dt in G[s]:
        if dist[t] > ds + dt:
            dist[t] = ds + dt
            heapq.heappush(que, (ds + dt, t))

print(dist[size-1])