n,m = map(int, input().split())
c = {}
k = []
l = 0
for i in range(m):
    a,b = map(int, input().split())
    c[a] = b
for j in c.keys():
    if n >= j:
        k.append(c[j])
if len(k) != 0:
    l = max(k)
print(n - l)