Validate InputBox Entry

validation up to the point that the user can click cancel and exit, also when nothing is entered a msgbox appears and for the 3rd inputbox a value greater than 0 must be entered.


Sub Trajectory() 'Trajectory macro
t0 = InputBox("Enter a value for the initial time(t0)")
'If t0 = "" Then MsgBox ("You must enter a value for t0!")
'Exit Sub
tf = InputBox("Enter a value for the final time(tf)")
'If tf = "" Then MsgBox ("You must enter a value for tf!")
'Exit Sub
Dt = InputBox("Enter a value for the time increment(Dt)")
'If Dt = "" Then MsgBox ("You must enter a value for Dt!")
'Exit Sub
'If Dt = 0 Then MsgBox ("You must have a valid increment")
x0 = Val( Range("F4"))
v0 = Val(Range("F5"))
g = Val(Range("F6"))
y0 = Val(Range("F7"))
q0 = Val(Range(" F8"))
Selection.Formula = FILL_TABLE
End Sub