diff --git a/src/com/annimon/everlastingsummer/Parser.java b/src/com/annimon/everlastingsummer/Parser.java
index 87ed8f9..5a3683d 100644
--- a/src/com/annimon/everlastingsummer/Parser.java
+++ b/src/com/annimon/everlastingsummer/Parser.java
@@ -411,6 +411,16 @@ public final class Parser {
     }
     
     private Expression expression() {
+        return unary();
+    }
+    
+    private Expression unary() {
+        if (match(TokenType.MINUS)) {
+            return new ValueExpression( -primary().eval() );
+        }
+        if (match(TokenType.PLUS)) {
+            return primary();
+        }
         return primary();
     }