为啥RE

l,m = map(int,input().split())
lst = []
for i in range(l):
        lst.append(True)
for i in range(m):
        a,b = map(int,input().split())
        for j in range(a,b+1):
                lst[j] = False
num = 0
for i in range(l):
        if lst[i] == True:
                num += 1
print(num+1)