n,m=map(int,input().split())
a = [-1]*(n+1)

ans = 0
for i in range(m):
  s,c=map(int,input().split())
  if a[s] == -1 or a[s] == c:
    a[s] = c
  else:
    print(-1)
    exit()

if a[1] == 0:
  if n>1:
    print(-1)
    exit()
  else:
    print(ans)
else:
  ans = max(1, a[1])
  for i in range(2, n+1):
    ans = ans*10+max(0, a[i])
print(ans)
