diff --git a/parser/instruction.go b/parser/instruction.go
index 5915c52..b1fcce5 100644
--- a/parser/instruction.go
+++ b/parser/instruction.go
@@ -46,11 +46,6 @@ func (inst Instruction) SameOp(instTwo Instruction) bool {
 	return inst.operator == instTwo.operator
 }
 
-// Complement compares Instructions operator and operand
-func (inst Instruction) Complement(instTwo Instruction) bool {
-	return inst.SameOp(instTwo) && inst.operand+instTwo.operand == 0
-}
-
 // IsZeroOp returns true for ops that have left the pointer on a zero
 func (inst Instruction) IsZeroOp() bool {
 	return !(inst.operator == opAddDp ||
diff --git a/parser/instruction_test.go b/parser/instruction_test.go
index 80a23e2..99f0878 100644
--- a/parser/instruction_test.go
+++ b/parser/instruction_test.go
@@ -140,26 +140,3 @@ func TestSameOp(t *testing.T) {
 	}
 
 }
-
-func TestComplement(t *testing.T) {
-	opsList := []Opcode{
-		opAddDp,
-		opAddVal,
-	}
-
-	for row, rval := range opsList {
-		for col, cval := range opsList {
-			t.Run(fmt.Sprintf("%s-%s", opName[rval], opName[cval]), func(t *testing.T) {
-				for operand := range [6]int{} {
-					rinst := Instruction{rval, operand}
-					cinst := Instruction{cval, operand - 6}
-					want := row == col && operand == 3
-
-					if rinst.Complement(cinst) != want {
-						t.Errorf("testing %v vs %v want %v", rinst, cinst, want)
-					}
-				}
-			})
-		}
-	}
-}
diff --git a/parser/tokenise.go b/parser/tokenise.go
index 5612101..4711120 100644
--- a/parser/tokenise.go
+++ b/parser/tokenise.go
@@ -111,14 +111,16 @@ func Tokenise(input io.ByteReader) (program []Instruction, err error) {
 
 func evalFactors(program []Instruction) (pointers, factors []int, ok bool) {
 	ok = false
+	invert := false
 	collect := 0
 	pointers = make([]int, 0)
 	factors = make([]int, 0)
 	for _, inst := range program {
 		if inst.SameOp(NewInstruction('>')) {
 			collect += inst.operand
-		} else if collect == 0 && inst.Complement(NewInstruction('+')) {
-			ok = true
+		} else if collect == 0 && inst.SameOp(NewInstruction('-')) {
+			ok = inst.operand == 1 || inst.operand == -1
+			invert = inst.operand == 1
 		} else if inst.SameOp(NewInstruction('-')) {
 			pointers = append(pointers, collect)
 			factors = append(factors, inst.operand)
@@ -129,5 +131,10 @@ func evalFactors(program []Instruction) (pointers, factors []int, ok bool) {
 	if collect != 0 || !ok {
 		return pointers, factors, false
 	}
+	if invert {
+		for i := range len(factors) {
+			factors[i] = 0 - factors[i]
+		}
+	}
 	return
 }