n= input().split()
L = int(n[0])+1
k = int(n[1])
c = []
for _ in range(k):
    l, r = map(int, input().split())
    c.append((l, r))
remove = set()
for l,r in c:
    for i in range (l,r+1):
        remove.add(i)
trees = L-len(remove)
print(trees)