diff --git a/docs/features/differences.md b/docs/features/differences.md
index 98b3b7c3..02f755f8 100644
--- a/docs/features/differences.md
+++ b/docs/features/differences.md
@@ -40,4 +40,6 @@ The easiest way to test a feature is to [try it on your browser](https://pocketp
 6. `__ne__` is not required. Define `__eq__` is enough.
 7. Raw string cannot have boundary quotes in it, even escaped. See [#55](https://github.com/blueloveTH/pocketpy/issues/55).
 8. In a starred unpacked assignment, e.g. `a, b, *c = x`, the starred variable can only be presented in the last position. `a, *b, c = x` is not supported.
-9. A `Tab` is equivalent to 4 spaces. You can mix `Tab` and spaces in indentation, but it is not recommended.
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+9. A `Tab` is equivalent to 4 spaces. You can mix `Tab` and spaces in indentation, but it is not recommended.
+10. `%`, `&`, `//`, `^` and `|` for `int` behave the same as C, not python.
+11. `int` is of limited precision. For arbitrary sized integers, you may use `long` type explicitly.
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diff --git a/docs/features/long.md b/docs/features/long.md
new file mode 100644
index 00000000..cf8987d5
--- /dev/null
+++ b/docs/features/long.md
@@ -0,0 +1,22 @@
+---
+icon: dot
+title: Arbitrary Sized Integers
+---
+
+Unlike cpython, pkpy's `int` is of limited precision.
+In 32 bit platforms, it is 30 bit;
+in 64 bit platforms, it is 62 bit.
+
+For arbitrary sized integers, we provide a builtin `long` type, just like python2's `long`.
+`long` is implemented via pure python in [_long.py](https://github.com/blueloveTH/pocketpy/blob/main/python/_long.py).
+
+```python
+a = long(2)         # use long() to create a long explicitly
+print(a ** 1000)
+# 10715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376L
+```
+
+!!!
+This feature is still under development.
+Some operations are missing, and some operations are not optimized.
+!!!
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