n,m=map(int,input().split())
z=[1,n]
es=[]
for i in range(m):
  a,b=map(int,input().split())
  es+=[(a,b)]
  z+=[a,b]
z=sorted(set(z))
d={v:i for i,v in enumerate(z)}
l=len(z)
e=[[] for i in range(l)]
for i in range(l-1):
  e[i]+=[(i+1,(z[i+1]-z[i])*2)]
for a,b in es:
  e[d[a]]+=[(d[b],2*b-2*a-1)]
X=1<<60
v=[X]*l
v[0]=0
q=[(v[0],0)]
from heapq import heappush,heappop
while len(q)>0:
  sc,sp=heappop(q)
  if sc>v[sp]:
    continue
  for tp,tc in e[sp]:
    if v[tp]>sc+tc:
      v[tp]=sc+tc
      heappush(q,(v[tp],tp))
print(v[-1])