# C
N, M = list(map(int, input().split()))

flag = 0
ans = [0]*M
ans_flag = [0]*M

sc = [list(map(int, input().split())) for _ in range(M)]
for i in range(M):
    s, c = sc[i]
    if ans_flag[s-1] == 1:
        if ans[s-1] != c:
            flag = 1
            break
    ans_flag[s-1] = 1
    ans[s-1] = c

if flag == 1:
    print("-1")
else:
    if ans[0] == 0 and ans_flag[0]==1:
        print("-1")
    else:
        if ans[0]==0:
            ans[0] = 1
        for i in ans:
            print(i, end="")


    
