from collections import defaultdict
from heapq import heapify, heappop, heappush

n, m = map(int, input().split())
Edge = []
p = set([0, n - 1])
for _ in range(m):
    a, b = map(int, input().split())
    Edge.append((a - 1, b - 1))
    p.add(a - 1)
    p.add(b - 1)
sortP = sorted(p)
l = len(sortP)

idx = 0
H = defaultdict(int)
for cp in sortP:
    H[cp] = idx
    idx += 1
route = defaultdict(list)
for a, b in Edge:
    route[H[a]].append((H[b], 2 * (b - a) - 1))
for i in range(l - 1):
    route[i].append((i + 1, 2 * (sortP[i + 1] - sortP[i])))

INF = 10**18
DP = [INF for _ in range(l)]
H = [(0, 0)]
heapify(H)
while H:
    ccost, cp = heappop(H)
    if DP[cp] <= ccost:
        continue
    DP[cp] = ccost
    if cp == l - 1:
        print(DP[l - 1])
        break
    for np, dcost in route[cp]:
        heappush(H, (ccost + dcost, np))