Random problem

I have four movie clips (mc1, mc2, mc3, mc4). Upon clicking a button (btn), two of the mc’s should disappear. Which two will disappear should be random. But there is one condition. There is a variable (say, k). An mc, mc* can disappear only if i != k.

I tried this in various ways, but in each way either an mc is taken twice or the value of i is more than 4 or less than 0.

I can see only one fool-proof method to do this but it is lengthy and unprofessional. Any help would be appreciated.