As the error message has already suggested you, the input is not correct and it can not be converted in to an int.
The best thing you can do is put a break-point on this line of code and then see what is the value you have selected comboBox1.
As per your comments, you are having values like 1-3, 5-6. If you want both values, you will have to have it in two different integer variables as shown below
string[] splitValues = Int32.Parse(comboBox1.SelectedItem.Split('-');
int firstValue = Int32.Parse(splitValues[0]);
int firstValue = Int32.Parse(splitValues[1]);
Hope that helps.
Milind