program enter_data2
    implicit none
    integer::n
    real::mark,sum,average
    sum = 0.0
    n = 0
    
    print *, "input Number. a number less than zero is for end of data."
    do
        read *,mark
        if (mark<0.0) exit
        sum = sum+mark
        n = n+1
    end do
    average = sum/real(n)
    print *,'number of data=',n
    print *,'average=',average
end program enter_data2