The Power Cube is used as a stash of Exotic Power. There are $n$ cities numbered $1, 2, \dots, n$ where allowed to trade it. The trading price of the Power Cube in the $i$-th city is $a_i$ dollars per cube. Noswal is a foxy businessman and wants to quietly make a fortune by buying and reselling Power Cubes. To avoid being discovered by the police, Noswal will go to the $i$-th city and choose exactly one of the following three options on the $i$-th day:
1. spend $a_i$ dollars to buy a Power Cube
2. resell a Power Cube and get $a_i$ dollars if he has at least one Power Cube
3. do nothing
Obviously, Noswal can own more than one Power Cubes at the same time. After going to the $n$ cities, he will go back home and stay away from the cops. He wants to know the maximum profit he can earn. In the meanwhile, to lower the risks, he wants to minimize the times of trading (include buy and sell) to get the maximum profit. Noswal is a foxy and successful businessman so you can assume that he has infinity money at the beginning.