n,m = map(int,input().split())
a = [list(map(int,input().split()))for i in range(m)]

for i in range(10**n):
    tmp = str(i)
    if len(tmp) == n:
        f = True
        for s,c in a:
            if tmp[s-1] != str(c):
                f = False
                break
            if f :
                print(i)
                exit()
print(-1)