As the war between Israel and Iran enters its second week, there’s a specific kind of math that will determine just how much longer it can go — how many long-range missiles Iran has versus how many missile interceptors Israel has to shoot them down.

Over the course of several days, Iran launched more than 400 missiles and hundreds of drones in retaliation for Israel’s surprise strikes last week, according to the Israeli military. Israel has managed to shoot most of them down — although several have hit — but as the barrages from Iran continue, Israel is using interceptors faster than it can make them.

“Whenever you’re talking about somebody shooting big ballistic missiles at you, you pay real close attention to the clock in terms of how long you can shoot back,” says Tom Karako, director of the Missile Defense Project at the Center for Strategic and International Studies.

Karako says his biggest worry right now is that, eventually, Israel’s missile interceptors could run out: “That would be a bad day. That would be a very bad situation.”

Israeli officials won’t comment on how many interceptors it has left, as that would potentially give Iran much-coveted information to gain an advantage. The Israeli military told NPR it “is prepared and ready to handle any scenario,” when asked about its extended ability to intercept long-range missiles.