n,m = map(int,input().split())
sc = [list(map(int,input().split())) for _ in range(m)]

ans = [0]*n
judge_s = 0
judge_c = 0
sc = sorted(sc)

if m == 0:
    if n == 1:
        ans = 0
    else:
        ans = 10**(n-1)
else:
    for i in range(m):
        if judge_s == sc[i][0] and judge_c != sc[i][1]:
            ans = -1
            break
        if n != 1 and sc[i] == [1,0]
            ans = -1
            break
        judge_s = sc[i][0]
        judge_c = sc[i][1]
        ans[sc[i][0]-1] = sc[i][1]


    if ans != -1:
        ans_ = ans.copy()
        ans = 0
        for i in range(n):
            ans += ans_[i]*10**(n-i-1)
        
        if ans_[0] == 0 and n != 1:
            ans += 10**(n-1)
            
    
print(ans)