x = readInt()
#PREMISES FOR ATTACHED PROOF, IF ANY: 
# True
#PREMISES FOR NEXT LINE: 
sum = 0
#PREMISES FOR ATTACHED PROOF, IF ANY: 
# (sum == 0)
#PREMISES FOR NEXT LINE: 
# (sum == 0)
i = 0
#PREMISES FOR ATTACHED PROOF, IF ANY: 
# (i == 0)
# (sum == 0)
assert (sum == (x * i))  # UNABLE TO VERIFY
"""{ 1.?? sum == x * i   algebra  }"""
#PREMISES FOR NEXT LINE: 
# (sum == (x * i))
sum = sum + x
#PREMISES FOR ATTACHED PROOF, IF ANY: 
# (sum == (sum_old + x))
# (sum_old == (x * i))
"""{ 1.OK  sum == sum_old  + x   premise
     2.OK  sum_old  == x * i     premise
     3.OK  sum == (x*i) + x      algebra 2 1
}"""
#PREMISES FOR NEXT LINE: 
# (sum == ((x * i) + x))
i = i + 1
#PREMISES FOR ATTACHED PROOF, IF ANY: 
# (i == (i_old + 1))
# (sum == ((x * i_old) + x))
assert (sum == (x * i))  # UNABLE TO VERIFY
"""{ 1.OK i == i_old + 1      premise
     2.OK sum == (x * i_old) + x   premise
     3.?? sum == x * i             algebra 2 1   # doesn't work!  Try this:
     4.OK sum == x * (i_old + 1)   algebra 2
     5.OK sum == x * i             subst 1 4
}"""
#PREMISES FOR NEXT LINE: 
# (sum == (x * i))