N, M = map(int, input().split())
sc = []
tf = True
tff = False
for i in range(M):
  sc.append(list(map(int, input().strip().split())))
num = []
for i in range(N):
  num.append(0)
sc.sort()
if N == 1 and sc[0][0] == 1 and sc[0][1] == 0:
  tff = True
for i in range(M-1):
  if sc[i][0] == sc[i+1][0] and sc[i][1] != sc[i+1][1]:
    tf = False
for i in sc:
  if i[0] == 1 and i[1] == 0:
    tf = False
  else:
    num[i[0]-1] = i[1]
if tf and num[0] == 0:
  num[0] = 1
string = ""
for i in num:
  string += str(i)
if tff == True:
  print(0)
elif tf == True:
  print(string)
else:
  print(-1)