import sys
ANS = []
N, M = map(int, input().split())
for j in range(N):
  ANS.append(0)
for i in range(M):
  s, c = map(int, input().split())
  ANS[s-1] = c
 
if ANS[0] == 0:
  print(-1)
  sys.exit()
 
NEW = 0
for i in range(N):
  NEW += ANS[-i-1] * (10**(i+1))
 
print(NEW)