N,M = map(int,input().split())

s = set()
from collections import defaultdict
G = defaultdict(list)
for _ in range(M):
    a,b = map(int,input().split())
    G[a].append((b,2 * b - 2 * a - 1))
    s.add(a)
    s.add(b)
s.add(1)
s.add(N)
l = sorted(s)
D = {v:i for i,v in enumerate(l)}

n = len(l)
for i in range(n - 1):
    G[l[i]].append((l[i+1],2 * (l[i+1] - l[i])))
                   
inf = 10 ** 10
dist = [inf] * n
dist[0] = 0
import heapq
q = [(0,1)]
while q:
    d,now = heapq.heappop(q)
    if dist[D[now]] < d:continue
    for v,c in G[now]:
        if dist[D[v]] > d + c:
            dist[D[v]] = d + c
            heapq.heappush(q,(d + c,v))
print(dist[-1])