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

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

if n == 1:
    a = [str(i) for i in range(10)]
elif n == 2:
    a = [str(i) for i in range(10:100)]
elif n == 3:
    a = [str(i) for i in range(100:1000)]

ans = []

for i in a:
    b = True
    for j in s:
        if int(i[j[0]-1]) != j[1]:
            b = False
            break
    if b:
        ans.append(i)

if len(ans) == 0:
    print(-1)
else:
    print(min(ans))

