diff --git a/compiler.rkt b/compiler.rkt
index 1385616..c342d31 100644
--- a/compiler.rkt
+++ b/compiler.rkt
@@ -15,10 +15,12 @@
 ;; flips the arguments of +. -Jeremy
 (define (flip-exp e)
   (match e
+    [(Int n) e]
     [(Var x) e]
     [(Prim 'read '()) (Prim 'read '())]
     [(Prim '- (list e1)) (Prim '- (list (flip-exp e1)))]
-    [(Prim '+ (list e1 e2)) (Prim '+ (list (flip-exp e2) (flip-exp e1)))]))
+    [(Prim '+ (list e1 e2)) (Prim '+ (list (flip-exp e2) (flip-exp e1)))]
+    [(Prim '- (list e1 e2)) (Prim '- (list (flip-exp e1) (flip-exp e2)))]))
 
 (define (flip-Lint e)
   (match e
@@ -36,12 +38,18 @@
     [((Int n1) (Int n2)) (Int (fx+ n1 n2))]
     [(_ _) (Prim '+ (list r1 r2))]))
 
+(define (pe-sub r1 r2)
+  (match* (r1 r2)
+    [((Int n1) (Int n2)) (Int (fx- n1 n2))]
+    [(_ _) (Prim '- (list r1 r2))]))
+
 (define (pe-exp e)
   (match e
     [(Int n) (Int n)]
     [(Prim 'read '()) (Prim 'read '())]
     [(Prim '- (list e1)) (pe-neg (pe-exp e1))]
-    [(Prim '+ (list e1 e2)) (pe-add (pe-exp e1) (pe-exp e2))]))
+    [(Prim '+ (list e1 e2)) (pe-add (pe-exp e1) (pe-exp e2))]
+    [(Prim '- (list e1 e2)) (pe-sub (pe-exp e1) (pe-exp e2))]))
 
 (define (pe-Lint p)
   (match p
diff --git a/interp-Lint.rkt b/interp-Lint.rkt
index c32bc55..329915b 100644
--- a/interp-Lint.rkt
+++ b/interp-Lint.rkt
@@ -26,6 +26,10 @@
        (define v1 (interp-exp e1))
        (define v2 (interp-exp e2))
        (fx+ v1 v2)]
+      [(Prim '- (list e1 e2))
+       (define v1 (interp-exp e1))
+       (define v2 (interp-exp e2))
+       (fx- v1 v2)]
       ))
 
 (define (interp-Lint p)
@@ -55,6 +59,10 @@
          (define v1 ((interp-exp env) e1))
          (define v2 ((interp-exp env) e2))
          (fx+ v1 v2)]
+        [(Prim '- (list e1 e2))
+         (define v1 ((interp-exp env) e1))
+         (define v2 ((interp-exp env) e2))
+         (fx- v1 v2)]
         ))
 
     (define/public (interp-program p)