#!/usr/bin/python

def sum(x, y):
    return x + y

def minus(x, y):
    return x - y

def sum_minus( x = 1, y = 1, z = 1, w = 1):
    return  sum(x, y) * minus(z, w)

def main():
    print sum_minus(1, 3, 5, 7)

if __name__ == "__main__":
    main()