import sys
n,m = map(int,input().split())
c=[list(map(int,input().split())) for i in range(m)]

if n > 1:
  for i in range(m):
    if c[i][0] == 1 and c[i][1] ==0 :
      print(-1)
      sys.exit()
  
for i in range(m):
  for j in range(m):
    if c[i][0] == c[j][0] and c[i][1] != c[j][1] :
      print(-1)
      sys.exit()
          
c1 = 0
c2 = 0
c3 = 0

for i in range(m):
  if c[i][0] == 1:
    c1 = c[i][1]
  elif c[i][0] == 2 :
    c2 = c[i][1]
  elif c[i][0] == 3 :
    c3 = c[i][1]

if n == 1 :
  print(c1)
elif n==2:
  print(c1 * 10 + c2)
elif n==3:
  print(c1 * 100 + c2 * 10 + c3)



