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

n, m = map(int, input().split())
Edge = defaultdict(list)
A = set()
for _ in range(m):
    a, b = map(int, input().split())
    Edge[a - 1].append((b - 1, 2 * b - 2 * a - 1))
    A.add(a - 1)
for c in Edge.keys():
    Edge[c].sort(key=lambda x: x[0])
A.add(n - 1)
sortA = sorted(A)
m = len(sortA)

INF = 10**18
DP = defaultdict(lambda: INF)
DP[0] = 0
H = [0]
seen = set()
heapify(H)
while H:
    cp = heappop(H)
    if cp == n - 1:
        print(DP[n - 1])
        break
    ccost = DP[cp]
    for np, dcost in Edge[cp]:
        if ccost + dcost > DP[np]:
            break
        DP[np] = ccost + dcost
        if np in seen:
            continue
        seen.add(np)
        heappush(H, np)
    idx = bisect_right(sortA, cp)
    for i in range(idx, m):
        np = sortA[i]
        if ccost + 2 * (np - cp) > DP[np]:
            break
        DP[np] = ccost + 2 * (np - cp)
        if np in seen:
            continue
        seen.add(np)
        heappush(H, np)