javascript validation for allow -1 and all positive numbers in text box

I am trying to validate a text box to allow all positive numbers including -1 in it.

i tried this, which will work for allowing only positive numbers

  function allownumbers(e, txtBox) {
         var key;
 
         key = (e.keyCode) ? e.keyCode : e.charCode;
         if (e.charCode == 0) {
             return true;
         }
         if ((key < 48 || key > 57) && (key != 46) && (key != 44)) {               
             return false;                
         }
         if (key == 46) {
             if ((txtBox.value).indexOf('.') != -1) {
                 return false;
             }
         }
         if (key == 44) {
             if ((txtBox.value).indexOf(',') != -1) {
                 return false;
             }
         }
         return true;
     }
 

But how to allow -1(only) with all positive numbers
Thanks in advance

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