import sys

a = sys.stdin.readline().split()
L, M = int(a[0]), int(a[1])
hash = dict.fromkeys(range(L + 1), 1)
for _ in range(M):
    tree = sys.stdin.readline().split()
    s, e = int(tree[0]), int(tree[1])
    for i in range(s,e+1):
        if hash[i] == 1:
            hash[i] = 0
sum=sum(hash.values())
print(sum)