from collections import deque
n = int(input())
d = deque([i for i in range(n+1)])
m = int(input())
for _ in range(m):
  b, c = map(int, input().split())
  c %= b
  for i in range(len(d)):
    x = d.popleft()
    if x % b == c:
      d.append(x)
print(d[0] if d else "NaN")