python RE0分 求助（样例是对的）

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)