Calculate negative prices properly

Cents should be negative as well.

src/PriceEditor.js

@@ -2,6 +2,12 @@ import React, { useState } from "react";
 import { Form } from "react-bootstrap";
 
 function str2price(str) {
+  let negative = false;
+  if (str[0] === "-") {
+    str = str.substring(1);
+    negative = true;
+  }
+
   let value = str.split(",");
   let cents = 0;
 
@@ -27,7 +33,11 @@ function str2price(str) {
     }
   }
 
-  return parseInt(value[0]) * 100 + cents;
+  let price = parseInt(value[0]) * 100 + cents;
+  if (negative) {
+    price = -price;
+  }
+  return price;
 }
 
 function price2str(price) {