n = int(input())
m = int(input())
BC = []
memo = dict()

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

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

for i in range(n + 1):
    if N[i] == m:
        print(i)
        exit()
print("NaN")