diff --git a/src/lib/expr.test.ts b/src/lib/expr.test.ts
index a4917c9..e998605 100644
--- a/src/lib/expr.test.ts
+++ b/src/lib/expr.test.ts
@@ -5,6 +5,7 @@ describe("expr", () => {
   describe("Expr does math", () => {
     test.each([
       ["1", "1"],
+      ["-1", "-1"],
       ["1 + 2", "3"],
       ["1 - 2", "-1"],
       ["1 * 2", "2"],
diff --git a/src/lib/expr.ts b/src/lib/expr.ts
index 4af04a1..4bd6b09 100644
--- a/src/lib/expr.ts
+++ b/src/lib/expr.ts
@@ -73,6 +73,12 @@ const Operators: TokenHandler[] = [
     token: NUMBER_TOKEN,
     parse: (left, matched) => ({ value: Number(matched) }),
   },
+  {
+    leftBindingPower: -1,
+    token: MINUS_TOKEN,
+    parse: (left, matched, parser) =>
+      map(parser.parseSubExpression(0), (right) => ({value: -right})),
+  },
   makeInfixOp(PLUS_TOKEN, 10, 11, (left, right) => ({ value: left + right })),
   makeInfixOp(MINUS_TOKEN, 10, 11, (left, right) => ({ value: left - right })),
   makeInfixOp(TIMES_TOKEN, 10, 11, (left, right) => ({ value: left * right })),