Thanks BigDod, finally I am done. What do you think, is it complete (for algorithm)? Here it is:
Do
$a = InputBox("Side A", "Length of side A (cm)", "Numbers Only")
If @error = 1 Then Exit
If StringIsAlpha($a) Then MsgBox(16, "Numbers Only!", "Please enter numbers only.")
If $a < 0 Or @error Then MsgBox(16, "Error!", "Invalid value.")
Until $a > 0 And Not @error
Do
$b = InputBox("Side B", "Length of side B (cm)", "Numbers Only")
If @error = 1 Then Exit
If StringIsAlpha($B) Then MsgBox(16, "Numbers Only!", "Please enter numbers only.")
If $b < 0 Or @error Then MsgBox(16, "Error!", "Invalid value.")
Until $b > 0 And Not @error
SoundPlay(@WindowsDir & "\media\tada.wav")
MsgBox(64, "Area and Perimeter", "Area: " & $a * $b & " cm²" & @CRLF & "Perimeter: " & (2 * $a) + (2 * $B) & " cm")