diff --git a/main.c b/main.c
index e03d567..d39256a 100644
--- a/main.c
+++ b/main.c
@@ -71,6 +71,14 @@ static struct Object *expr();
 static struct Object *parens();
 static struct Object *parens_part();
 
+/********
+ * Eval *
+ ********/
+
+static struct Object *eval(struct Object *program);
+
+static struct Object *func_sum(struct Object *numbers);
+
 /*******************
  * Implementations *
  *******************/
@@ -89,6 +97,7 @@ int main()
     }
 
     struct Object *const program = parse();
+    struct Object *const result = eval(program);
 
     exit(EXIT_SUCCESS);
 }
@@ -357,7 +366,7 @@ struct Object *expr()
     case STATE_NUM:
     {
         struct Object *const object = prepare_object(TYPE_NUMBER);
-        object->s = tokens->val;
+        object->i = atoll(tokens->val);
         shift_token();
         return object;
     }
@@ -391,3 +400,60 @@ struct Object *parens_part()
 
     return object;
 }
+
+/********
+ * Eval *
+ ********/
+
+struct Object *eval(struct Object *const program)
+{
+    if (program->type != TYPE_PAIR) {
+        error("eval expects pair");
+        return NULL;
+    }
+
+    if (!program->a || program->a->type != TYPE_ATOM) {
+        error("eval expects atom");
+        return NULL;
+    }
+
+    if (strcmp(program->a->s, "sum") != 0) {
+        error("unknown func");
+        return NULL;
+    }
+
+    return func_sum(program->b);
+}
+
+struct Object *func_sum(struct Object *const numbers)
+{
+    struct Object *const object = prepare_object(TYPE_NUMBER);
+    object->i = 0;
+
+    if (numbers) {
+        if (numbers->type == TYPE_NUMBER) {
+            object->i = numbers->i;
+        } else if (numbers->type == TYPE_PAIR) {
+            if (numbers->a->type != TYPE_NUMBER) {
+                error("type error");
+                return NULL;
+            }
+
+            object->i = numbers->a->i;
+
+            struct Object *b = eval(numbers->b);
+
+            if (numbers->b->type != TYPE_NUMBER) {
+                error("type error");
+                return NULL;
+            }
+
+            object->i += b->i;
+        } else {
+            error("type error");
+            return NULL;
+        }
+    }
+
+    return object;
+}