import sys
l,m=map(int,sys.stdin.readline().split())
a=[1]*(l+1)
for _ in range(m):
    li,ri=map(int,sys.stdin.readline().split())
    for i in range(li,ri+1):
        a[i]=0
count=0
for i in a:
    if i ==1:
        count+=1
print(count)