You are a top secret agent trying to get into a enemy hide out. When you arrive at the enemy base, you see two guards at the door. You could take them out but that would make a ruckus and the enemies will know you are here, thus you wait in a bush. You see a man walk up towards the guards.
"1!" yells the guard in a thundering voice, so loud that you can almost hear the exclamation mark. The man replies with "1" and is allowed him in.
As the man walks in and the door closes behind him, another person walks up to the guards. "2!" yells the guard, again with a resounding exclamation mark. The person then replied with "2".
"These aren't the smartest bad guys I've ever went against" you say to yourself as you start walking towards the guards. The guard yells "3!", and you reply with confidence "3". The guards look at each other and then shoot you.
As you lie there bleeding, you see another person walk up to the guards. The guard yells out "0!" and the person replies with "1". The answer hits you as you slowly lose consciousness.
What is the answer to their password system?