static long minFloors(long[] buildings, int m) {
    		//sort buildings
    		Arrays.sort(buildings);
    		//maintain a window of size m, compute the minCost of each window, update minCost along the way as the final result
    		long minCost = Long.MAX_VALUE;
    		for(int i = 0; i <= buildings.length-m; i++){
    			long heightToMatch = buildings[i+m-1];
    			if(heightToMatch == buildings[i]) return 0;//if the last building's height equals the first one, that means the whole window if of the same size, we can directly return 0
    			long thisCost = 0;
    			for(int j = i+m-1; j >= i; j--){
    				thisCost += heightToMatch - buildings[j];
    			}
    			minCost = Math.min(minCost, thisCost);
    		}
    		return minCost;
    	}