from collections import defaultdict
from heapq import heappop, heappush
from itertools import pairwise
from math import inf


def main():
    N, M = map(int, input().split())
    graph = defaultdict(dict)
    roads_edge = set((0, N - 1))
    for _ in range(M):
        A, B = map(int, input().split())
        roads_edge.add(A - 1)
        roads_edge.add(B - 1)
        graph[A - 1][B - 1] = 2 * (B - A) - 1

    roads_edge = sorted(roads_edge)
    for prev_n, cur_n in pairwise(roads_edge):
        graph[prev_n][cur_n] = min(
            2 * (cur_n - prev_n), graph[prev_n].get(cur_n, inf))

    distance = {edge: inf for edge in roads_edge}
    distance[0] = 0
    queue = [(0, 0)]
    while queue:
        c_d, c_n = heappop(queue)
        if distance[c_n] < c_d:
            continue
        for n_n, n_d in graph[c_n].items():
            if distance[n_n] > n_d + c_d:
                distance[n_n] = n_d + c_d
                heappush(queue, (n_d + c_d, n_n))
    print(distance[N - 1])


if __name__ == "__main__":
    main()