N, M = map(int, input().split())

ans = ['*'] * N

for i in range(M):
    a, b = map(int, input().split())

    if ans[a-1] == '*':
        ans[a-1] = str(b)
    elif int(ans[a-1]) < b:
        ans[a-1] = str(b)

for i in range(M):
    if ans[i] == '*' or ans[i] == '0':
        print(-1)
        break
    else:
        print(''.join(ans).replace('*', '0'))
        break