n = int(input())
m = int(input())
BC = []
dic = {}

for i in range(m):
    b,c = map(int,input().split())
    c %= b
    if b in dic and dic[b] != c:
        print("NaN")
        exit()
    dic[b] = c
    BC.append([b,c])

nums = [0]*(n+1)
for b,c in BC:
    for i in range(c,n+1,b):
        nums[i] += 1

le = len(BC)
for i in range(n+1):
    if nums[i] == le:
        print(i)
        exit()
print("NaN")