diff --git a/modules-examples/books.js b/modules-examples/books.js
new file mode 100644
index 0000000..6f78ebd
--- /dev/null
+++ b/modules-examples/books.js
@@ -0,0 +1,48 @@
+export function isValidIsbn13(str) {
+    let sum, digit, check, i;
+
+    str = str.replace(/[^0-9X]/gi, '');
+
+    if (str.length !== 13) {
+        return false;
+    }
+    sum = 0;
+    for (i = 0; i < 12; i++) {
+        digit = parseInt(str[i]);
+        if (i % 2 === 1) {
+            sum += 3 * digit;
+        } else {
+            sum += digit;
+        }
+    }
+    check = (10 - (sum % 10)) % 10;
+    return (check === str[str.length - 1]);
+}
+
+export function isValidIsbn10(str) {
+    let sum, digit, weight, check, i;
+
+    str = str.replace(/[^0-9X]/gi, '');
+
+    if (str.length !== 10) {
+        return false;
+    }
+    weight = 10;
+    sum = 0;
+    for (i = 0; i < 9; i++) {
+        digit = parseInt(str[i]);
+        sum += weight * digit;
+        weight--;
+    }
+    check = (11 - (sum % 11)) % 11
+    if (check === 10) {
+        check = 'X';
+    }
+    return (check === str[str.length - 1].toUpperCase());
+
+}
+
+
+export function isValidIsbn(str) {
+    return isValidIsbn13(str) || isValidIsbn10(str);
+}
\ No newline at end of file
diff --git a/modules-examples/stats.js b/modules-examples/stats.js
index 81c07fb..8ceb4bd 100644
--- a/modules-examples/stats.js
+++ b/modules-examples/stats.js
@@ -1,3 +1,5 @@
+
+export const INFINITY = 30;
 export function average(lst){
     if(lst.length < 1){
         return NaN;