# coding: utf-8
# Your code here!

n,m = map(int,input().split())
a = [-1 for i in range(n)]
flag = 1
for i in range(m):
    b,c = map(int,input().split())
    b -= 1
    if a[b] != -1 and a[b] != c:
        flag = 0
        print("-1")
        break
    else:
        a[b] = c

if flag == 1:
    if a[0] == -1:
        print("-1")
    elif a[0] == 0:
        if n == 1:
            print(0)
        else:
            print(-1)
    else:
        ans = 0
        for i in range(n):
            if a[i] != -1:
                ans += a[i] * (10**(n-1-i))
        print(ans)