MinimumAmount

    static int calculate_amount(int[] prices){
        int totalAmount = 0;

        //empty array, nothing to buy
        if(prices.length < 1){
            return totalAmount;
        }


        int minPrice = prices[0];
        totalAmount += prices[0];

        for (int i = 1; i < prices.length; i++) {
            //get the value of the item with the discount
            int currentPrice = prices[i] - minPrice;

            //if the min price is bigger than current price, get the item for free
            if(currentPrice < 0){
                currentPrice = 0;
            }

            //ad it to the total amount
            totalAmount += currentPrice;

            // get current price if it is lower than previous min
            minPrice = minPrice > prices[i] ? prices[i] : minPrice;
        }

        return totalAmount;
    }