l,m = input().split()
a = [input() for _ in range(int(m))]
b = [1]*(int(l)+1)
# print(len(b))
for i in range(int(m)):
    for j in range(int(a[i].split()[0]),int(a[i].split()[1])+1):
        b[j]=0
count = 0
for c in range(len(b)):
    if b[c]==1:
        count+=1
print(count)