如何减去在python中设置的两个字符串?

我是python的新手,正在编写我的第一个程序。我正在使用PyCharm。对于我的python类,这是与计算机的简单对话。我在最后一行减去两个字符串的代码上遇到麻烦。请帮忙!代码如下:

print('Hello World!')
print('My name is Optimus Prime!')
print('What is your name?') # ask for their name
myName = input()
print('Hi, ' + myName + ' lets have some fun!')
print('What is your favorite sport?:') # ask for their favorite sport
favorite_sport = input()
print(favorite_sport + ' is awesome!')
print('Mine is dismantling decepticons. So much fun!')
print('What is your favorite team?') # ask for their favorite team
favorite_team = input()
print('How many players are on a baseball team?') # ask the number of players on a baseball team
number_of_players = input()
print('How many field positions are there in baseball?') # ask the number of field positions in   baseball
field_positions = input()
print('If there are ' + number_of_players + ' players on a team')
print(' and ' + field_positions + ' field positions')
print('then, ' (number_of_players - field_positions) ' are left in the dugout.')

我该怎么做呢?