import sys
from collections import defaultdict, deque
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)
E = []
mx = 0

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)
  E.append(a-1)
  E.append(b-1)
  mx = max(mx, b-1)
if n-1 not in st[mx]:
  G[mx].append((n-1, (n-1 - mx)*2))

E.sort()

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

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