Rope Assistant

Take a deck of cards and lay them in a horizontal row on the table. The cards must be in the open so that the next card covers half the previous one. Count the eighteenth card, for example, it's 3 hearts. Take the rope of a long no less than the resulting number of cards and tie the knot on the eighteenth card. After that, fold the cards into the pack and put the rope in your pocket. This was the preparation for the execution of the focus. And now the focus itself. Take a deck of cards and give it to any viewer to shuffle, then take the deck and put all the cards in a horizontal row in an open view so that the subsequent card closes half of the previous one. After that, give a piece of paper and a pen to the viewer, and ask the viewer to do a series of mathematical calculations, resulting in a map that you will find with your rope. Ask the viewer:

Write a four-digit number, for example 8125;
Write the number from right to left 5218;
Subtract a smaller number from the larger and write a total of 8125-5218 = 2907;
Add the digits of the number, which will be, for example, 2 + 9 + 0 + 7 = 18.

Remember, the result will always be the same number 18. Now remove the rope from your pocket and attach it to the cards in order to accidentally find the card under the number that was obtained.