from collections import defaultdict

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

V=[1,N]
E=defaultdict(list)
for i in range(M):
    A,B=map(int,input().split())

    V.append(A); V.append(B)
    E[A].append(B)

V=sorted(set(V)); L=len(V)
V_ind={v:i for i,v in enumerate(V)}

inf=float("inf")
DP=[inf]*L; DP[0]=0
for i in range(L-1):
    DP[i+1]=min(DP[i+1],DP[i]+2*(V[i+1]-V[i]))
    for b in E[V[i]]:
        DP[V_ind[b]]=min(DP[V_ind[b]], DP[i]+2*(b-V[i])-1)

print(DP[-1])