WA问题求助（Python）

程序在洛谷和pythonIDLE上都可以运行输出结果也一致，但是答题后提交测评无法通过，WA报错，求大佬解答一下

One = input()
One = list(map(int, One.split()))
Tree = '1' * (One[0] + 1)
for i in range(One[1]):
    Size = input()
    Size = list(map(int, Size.split()))
    Tree = Tree[:Size[0]:1] + '0' * (Size[1] - Size[0]) + Tree[Size[1] + 1::1]
print(int(Tree.count('1')))