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

ans = ['0'] * n
flag = [0] * n

for s, c in s_li:
    if flag[s - 1] == 0:
        ans[s - 1] = '{}'.format(c)
        flag[s - 1] = 1
    elif flag[s - 1] == 1 and ans[s - 1] == '{}'.format(c):
        continue
    else:
        ans = ['-1']
        break

ans = int(('').join(ans))

if n != 1 and (ans // 10**(n - 1)) == 0:
    ans = -1

print(ans)