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

a = [0]*n

for i in range(m):
    s, c = map(int, input().split())
    if a[s-1] == 0:
        a[s-1] = c
    elif a[s-1] != c:
        print(-1)
        exit()
if n != 1 and a[0] == 0:
    print(-1)
    exit()
for i in range(n):
    print(a[i], end="")
