n,m=map(int, input().split())
l = [None]*n
msg=''
for _ in range(m):
    s, c = input().split()
    index = int(s)-1
    if l[index] is None:
        l[index] = c
    else:
        if l[index] == c:
            continue
        else:
            msg=-1
            break
if l[0] is None:
    l[0]='1'
l = list(map(lambda x: '0' if x is None else x, l))
if msg == -1:
    print(msg)
elif l[0] == '0' and len(l) != 1:
    print(-1)
else:
    print(msg.join(l))
