警示后人 WA on#5

ll C(ll x,ll y){
	if(y<x) return 0; 
	//baoli
	ll ret = 1;
	rfor(i,y,y-x+1){
		ret = ret*i%MOD;// ret = ret*(i%MOD)%MOD;
	}
	rfor(i,x,1){
		ret = ret*inv[i]%MOD;
	}
	return ret%MOD;
}