# Put 1 grain of rice on the first square of a chess board, 2 on the second
# 4 on the third, 8 on the fourth, and so forth. Find how many grains of rice
# are on the board after you get to n squares
#
# Matt Bishop, MHI 289I, Winter 2019
#
try:
n = int(input("Number of squares? "))
except:
print("Need a positive integer!")
else:
# sanity check input
if n < 1:
print("Your integer must be positive!")
else:
# good to go!
# total = number of grains of rice on board
# as n is at least 1, there will be at least
# 1 grain of rice
total = 1
# x = number of grains of rice on current square
x = 1
# now add up the rice grains
for i in range(1, n):
# going to the next square, so we double the
# number of grains of rice on the previous
# square to get the number on the current square
x = 2 * x
# now add it into the total
total = total + x
# done! print it out
print("On a chessboard of", n, "squares, there are", total, "grains of rice")