salary amount to be calculated

Question

 days                      textbox1  calendarimage (what month i choose that a partiuclat day will be displayed in the textbox1)
                                31

 excludesunday         textbox2  (exclude sunday reaming days to be displayed in the textbox2)                     26
                                

 salary       textbox3   textbox4   
                 1000


on textbox3 changed i write the following code
{
double days = textbox1.text;
double sundays = textbox2.text;
textbox4.text = convert. double (textbox3.text / days * sundays);
}


when i run

i get the follwing errors

cannot implicity convert type string to int;
opertor / cannot be applied to operands of type system.windows.forms.textbox and double


please send the code.help me.