program t
    integer::a(100),b,c(100),i
    read(*,*)b
    a(1)=0
    c=0
    do i=1,100
        a(i)=a(i)+1
        c(i)=a(i)+c(i)
        if (a(i)==b) exit
    end do
    write(*,*)c(b)
end program t