PROCEDURE positive_input()
{
    DISPLAY("Enter a positive number: ")
    x <- NUMBER(INPUT())
    REPEAT WHILE(x <= 0)
    {
        DISPLAY("Invalid Input!\n")
        DISPLAY("Enter a positive number: ")
        x <- NUMBER(INPUT())
    }
    RETURN x
}

PROCEDURE main()
{
    a <- positive_input()
    b <- positive_input()
    c <- (a + b) / 2
    DISPLAY("The midpoint is " + c)
}