import sys
input = sys.stdin.readline
from collections import *

N, M = map(int, input().split())
G = defaultdict(list)
l = [1, N]

for _ in range(M):
    u, v = map(int, input().split())
    G[u].append(v)
    l += [u, v]

l = list(set(l))
l.sort()
idx = defaultdict(int)

for i in range(len(l)):
    idx[l[i]] = i
    
dp = [10**18]*len(l)
dp[0] = 0

for i in range(len(l)):
    if i+1<len(l):
        dp[i+1] = min(dp[i+1], dp[i]+2*(l[i+1]-l[i]))
    
    for j in G[l[i]]:
        dp[idx[j]] = min(dp[idx[j]], dp[i]+2*j-2*l[i]-1)

print(dp[-1])