s = input()
s = s.split()
b = int(s[0])
f = int(s[1])
k = 1
su = 0
a = []
for i in range(999):
	a.append(0)
for i in range(f):
	k += 1
	c = int(s[k])
	k += 1
	d = int(s[k])
	for j in range(c,d+1):
		a[j] = 1
for i in range(b+1):
	if a[i] == 0:
		su += 1
print(su)