N, M = map(int, input().split())
q = [list(map(int, input().split())) for i in range(M)]
change = [0 for i in range(N)]

ans = list('0' * N)
for i in range(M):
  if change[q[i][0]-1] == 0:
  	ans[q[i][0] - 1] = q[i][1]
  	change[q[i][0]-1] = 1
  else:
    if ans[q[i][0] - 1] != q[i][1]:
      ans = -1
      break
      
res = ''
for i in range(len(ans)):
  res += str(ans[i])
  
if res[0] == '0':
  print(-1)
else:
  print(int(res))
