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

def check(x):
  x=str(x)#文字に変換
  if len(x)!=N:
    return False
  for s,c in SC:
    if x[s-1]!=str(c):
      return False
  return True

for i in range(1000):
  if check(i):
    print(i)
    exit()
print(-1)