The formula of the four algorithms: four integer mixed operations, the operation order is kept in mind; Multiply, add, add and subtract without brackets, multiply first and then subtract; The first stage and the second stage count, and the second stage counts before the first stage; With the change of bracket order, count the inner side first and then the outer side; Arithmetic is the most useful and easy to use properly.

That is, if there are no brackets, multiply first and then divide, and then add and subtract. If there are brackets, count what is inside the brackets first, and then what is outside the brackets.