import sys
from collections import defaultdict
from heapq import heapify, heappush, heappop
input = lambda: sys.stdin.readline().rstrip()
inf = float('inf')

def dijkstra(G: list, s: int) -> list:
  "Return dist from s. / O(|E|log|V|)"
  dist = defaultdict(lambda: inf)
  dist[s] = 0
  hq = [(0, s)]
  while hq:
    d, v = heappop(hq)
    if dist[v] < d:
      continue
    for x,c in G[v]:
      if dist[x] > d + c:
        dist[x] = d + c
        heappush(hq, (d+c, x))
  return dist

#  -----------------------  #

n, m = map(int, input().split())
G = defaultdict(list)
st = defaultdict(set)
L = set()

for _ in range(m):
  a, b = map(int, input().split())
  G[a-1].append((b-1, 2*b-2*a-1))
  st[a-1].add(b-1)
  L.add(a-1)
  L.add(b-1)
if n-1 not in L:
  L.add(n-1)

L = sorted(list(L))

for i in range(len(L)-1):
  now, nxt = L[i], L[i+1]
  dist = (L[i+1] - L[i]) * 2
  if nxt not in st[now]:
    G[now].append((nxt, dist))

dist = dijkstra(G, 0)
print(dist[n-1])