diff --git a/a8/lisp.cc b/a8/lisp.cc
index 576ea82..f4e982d 100644
--- a/a8/lisp.cc
+++ b/a8/lisp.cc
@@ -119,9 +119,15 @@ namespace a8
                  [] (const List& params) -> std::shared_ptr<Value>
                  {
                      double result = 0;
+                     int i = 0;
                      for (auto param : *params) {
                          Atom&& atom = std::any_cast<Atom>(param->value);
-                         result -= atom.val;
+                         if (i <= 0) {
+                             result = atom.val;
+                         } else {
+                             result -= atom.val;
+                         }
+                         ++i;
                      }
                      return std::make_shared<Value>(Atom(result));
                  });