import heapq as hq
import sys
input = sys.stdin.readline
INF = 10 ** 18

def dijkstra(route, start, finish, n):
	path = [INF] * n
	q = [(0, start)]
	while(q):
		d, v = hq.heappop(q)
		if(path[v] <= d):
			continue
		path[v] = d
		if(v == finish):
			break
		for nv, dist in route[v]:
			hq.heappush(q, (d + dist, nv))
	return path[finish]

n, m = map(int,input().split())
h = {};	st = set([])
edge = []
for _ in [0] * m:
	a, b = map(int,input().split())
	a -= 1;	b -= 1
	st.add(a);	st.add(b)
	edge.append((a, b))
lis = sorted(st)
N = len(lis)

cnt = 0
for i in lis:
	h[i] = cnt
	cnt += 1
route = [[] for _ in [0] * N]
for a, b in edge:
	route[h[a]].append((h[b], b - a - 1))
for i in range(N - 1):
	route[i].append((i + 1, lis[i + 1] - lis[i]))

ans = lis[N - 1] - dijkstra(route, 0, N - 1, N)
print(ans)