N, M = input().split(" ")
N = int(N)
M = int(M)
s_list = []
c_list = []
for i in range(M):
    si, ci = input().split(" ")
    s_list.append(int(si))
    c_list.append(int(ci))
num = ["0" for i in range(N)]
for si, ci in zip(s_list, c_list):
    if num[si-1] != "0" and num[si-1] != str(ci):
        print(-1)
        is_break = True
        break
    elif N > 1 and si == 1 and ci == 0:
        print(-1)
        is_break = True
        break
    else:
        num[si-1] = str(ci)
if not is_break:
    num = int("".join([val for val in num]))
    if num:
        print(num)
    else:
        print(0)