money,ticket=tuple(map(int,input().split()))
dict={}
for i in range(0,ticket):
    key,value=tuple(map(int,input().split()))
    dict[key]=value
# print(dict)
least_fee=fee=money
for key in dict:
    if money >= key:
        fee=money-dict[key]
    if least_fee > fee:
        least_fee=fee
print(least_fee)