diff --git a/lib/mathematics.c b/lib/mathematics.c
index 32a406b..b8d14d9 100644
--- a/lib/mathematics.c
+++ b/lib/mathematics.c
@@ -32,3 +32,7 @@ float mathematics_square_root(float number) {
 unsigned long long mathematics_factorial(unsigned long long number) {
   return number == 0 ? 1 : number * mathematics_factorial(number - 1);
 }
+
+int64_t mathematics_opposite(int64_t number) {
+  return number * -1;
+}
diff --git a/lib/mathematics.h b/lib/mathematics.h
index 722cd59..c13bcbb 100644
--- a/lib/mathematics.h
+++ b/lib/mathematics.h
@@ -66,4 +66,12 @@ float mathematics_square_root(float number);
  */
 unsigned long long mathematics_factorial(unsigned long long number);
 
+/**
+ * @brief Calulcates the opposite number (additive inverse).
+ *
+ * @param number
+ * @return int64_t
+ */
+int64_t mathematics_opposite(int64_t number);
+
 #endif