N = int(input())
M = int(input())
true = [set() for _ in range(M+1)]
true[0] = {i for i in range(N+1)}

for i in range(1,M+1):
  b,c = map(int,input().split())
  d = c%b
  
  for j in true[i-1]:
    if j % b == d:
      true[i].add(j)
if len(true[M]) == 0:
  print('NaN')
  exit()
print(min(true[M]))