PYTHON (Loops)

Boot camp consisted of an interesting descending ladder workout today. Participants did 21 exercises in the first round and three less in each round after that until they did 3 exercises in the final round. How many exercises did the participants do during the workout? (84 for testing purposes) Write the code so that it provides a complete, flexible solution toward counting repetitions. Ask the user to enter the starting point, ending point and increment (change amount).