line_b = list(map(int, input().split(' ')))
line=[]
num=[]
for i in range(line_b[0]):
  num+=[0]
for i in range(line_b[1]):
  line+=[list(map(int, input().split(' ')))]
if len(num)!=1:
  for i in range(line_b[1]):
    if num[line[i][0]-1]==0:
      num[line[i][0]-1]=line[i][1]
    elif num[line[i][0]-1]!=line[i][1]:
      num[0]=0
      break
  for i in range(line_b[1]):
    for j in range(line_b[1]):
      if line[i][0]==line[j][0] and line[i][1]!=line[j][1]:
        num[0]=0
  if num[0]==0:
    print(-1)
  else:
    number=0
    for i in range(line_b[0]):
      number+=num[i]*10**(line_b[0]-1-i)
    print("%d" %number)
else:
  for i in range(line_b[1]):
    if num[line[i][0]-1]==0:
      num[line[i][0]-1]=line[i][1]
  t=0
  for i in range(line_b[1]):
    for j in range(line_b[1]):
      if line[i][0]==line[j][0] and line[i][1]!=line[j][1]:
        t=1
  if t==1:
    print(-1)
  else:
    number=0
    for i in range(line_b[0]):
      number+=num[i]*10**(line_b[0]-1-i)
    print("%d" %number)