import sys
import heapq as hq
input = sys.stdin.readline
INF = 4611686018427387903

n, m = map(int, input().split())
edges = [[x - 1 for x in map(int, input().split())] for _ in [0] * m]

V = set([0, n - 1])
for x, y in edges:
    V.add(x)
    V.add(y)
N = len(V)
V = sorted(V)
di = { x : i for i, x in enumerate(V) }
graph = [[] for _ in [0] * N]
for i in range(N - 1):
    graph[i].append((i + 1, 2 * (V[i + 1] - V[i])))
for x, y in edges:
    graph[di[x]].append((di[y], 2 * y - 2 * x - 1))

dp = [INF] * N
que = [(0, 0)]
while(que):
    c, v = hq.heappop(que)
    if(dp[v] <= c):
        continue
    dp[v] = c
    if(v == N - 1):
        break
    for nv, nd in graph[v]:
        hq.heappush(que, (c + nd, nv))
print(dp[N - 1])