n,m = list(map(int,input().split()))
nlist = []
clist = []
for i in range (m):
    s,c = list(map(int,input().split()))
    nlist.append(s)
    clist.append(c)

for i in range (len(nlist)-1):
    for j in range (1, len(nlist)):
        if nlist[i] == nlist[j]:
            if clist[i] != clist[j]:
                print(-1)
                exit()

a = 1
b = 0
c = 0

for i in range (len(nlist)):
    if nlist[i] == 1:
        a = (clist[i])
    if nlist[i] == 2:
        b = (clist[i])
    if nlist[i] == 3:
        c = (clist[i])

if n == 1:
    print(a)
elif n == 2:
    if a == 0:
        print(-1)
    else:
        print(a,end = "")
        print(b)
elif n == 3:
    if a == 0:
        print(-1)
    else:
        print(a,end = "")
        print(b,end = "")
        print(c)