a = input().split()
days = int(a[0])
wars = int(a[1])
whole = [0]*(days+1)
current = [0, 0]
peace = [0, 0]

for i in range(wars):
    c = input().split()
    c[0] = int(c[0])
    c[1] = int(c[1])
    for j in range(c[0], c[1]+1):
        whole[j] = 1

while True:
    if whole.count(1) == 0:
        peace[0] = days
        peace[1] = 1
        break
    elif whole.count(0) > 0:
        for i in range(whole.index(0), days):
            current[1] = whole.index(0)
            if whole[i] == 0:
                current[0] += 1
            else:
                break
        if current[0] > peace[0]:
            peace[0] = current[0]
        for i in range(whole.index(1) - 1 - whole.index(0)):
            del whole[whole.index(0)]
    elif whole.count(0) == 0:
        break
if peace[0] != 0:
    print(peace)
else:
    print(-1)