n, m = map(int, input().split())
ans = [0]*n

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

    ans[s-1] = c
if ans[0] == 0:
    print(-1)
    exit()
print(*ans, sep="")
