n, m = map(int, input().split())
array = []
for i in range(m):
  alist = list(map(int, input().split()))
  array.append(alist)
 
flg = True
num = [0]*n
 
for i in range(m):
  if num[array[i][0]-1] == 0:
    num[array[i][0]-1] = array[i][1]
  else:
    if num[array[i][0]-1] != array[i][1]:
      flg = False
      break
if flg == True:
  num2 = 0
  if num[0] == 0:
    num[0] = 1
  for i in range(n):
    num2  = num2 + num[i] * (10**(n-i-1))
  print(num2)
else:
  print(-1)