diff --git a/README.md b/README.md
index 2c7cffd..f98c432 100644
--- a/README.md
+++ b/README.md
@@ -4,6 +4,7 @@ An arithmetic derivative calculator that uses these simple formulas:
 * 0' = 1' = 0;
 * if n is a prime, n' = 1;
 * if n = ab  (a and b > 1), n' = a'b + ab'
+* (-n)' = -(n')
 
 More about arithmetic derivative:
 https://en.wikipedia.org/wiki/Arithmetic_derivative