 Usually, we write mathematical expressions in infix notation. This means that operations involving two numbers are written as the first number, followed by the operator, followed by the second number. Polish notation is an alternative to infix notation. In Polish notation, the operator is put up front. It's also called prefix notation or forward Polish notation in contrast to reverse Polish notation, which puts the operator at the end. One of the advantages of Polish notation is that it eliminates the need for brackets. For instance, 5-1 plus 2 is written as multiply 5 plus 1-2 without any brackets. Let's evaluate a few Polish notation expressions to get a feel for how it works. Since we're expecting to get a single number from evaluating these, we're going to start with a box that's expecting a single number. If we were to end up with multiple numbers from evaluating a whole expression, that would mean that something is wrong with that expression. For this expression, we start with multiplication. Since multiplication isn't a number, we can't put that in the box. Instead, we will need to multiply two other expressions to get our final answer. The first of those expressions is just the number 5. To get the second one, we need to add. The first number to add is 1. The second is 2. Now, we can resolve the bottom value of the multiplication to 3. Let's try another more complicated expression. Divide minus 5A multiply 3 divide 510. It starts with division, so we need to figure out what two numbers to divide. Next, we encounter a minus sign, so let's figure out what to subtract. 5 and 8, 5 minus 8 equals negative 3, so that's the first number to divide. To determine the second number, we need to multiply 3 and another number. We need to divide to get the second number. 5 and 10 divide to get 0.5. Next, we multiply 3 and 0.5 to get 1.5. Divide those two numbers to get negative 2, our end result. Thanks for watching!