n,m = map(int,input().split())

ans = True
s = [-1]*3
for _ in range(m):
    a,b = map(int,input().split())
    a -= 1

    if (s[a] == -1) or (s[a] == b):s[a]= b
#すでに数字をいれている桁に別の数字が入る場合　
    else:ans = False


#最上桁が0
if s[0] == 0:
    ans = False

if ans :
    for i in range(1000):
        x = str(i)
        cnt = 0
        if len(s) == len(x):
            for dig in range(len(x)):
                    if s[dig] == int(x[dig]):
                        cnt+=1
                    elif s[dig] == -1:
                        cnt+=1
                    else:
                        continue
            
            if cnt == 3:
                print(i)
                break
        else:
            continue


else:
    print(-1)

