i = int(input())
count = int(0)
sum = int(0)
while count < i:
sum = sum + count
count = count +1
print(sum)
aSA9IGludChpbnB1dCgpKQpjb3VudCA9IGludCgwKQpzdW0gPSBpbnQoMCkKCndoaWxlIGNvdW50IDwgaToKICAgICAgIHN1bSA9IHN1bSArIGNvdW50CiAgICAgICBjb3VudCA9IGNvdW50ICsxIAoKcHJpbnQoc3VtKQ==