diff --git a/lib/common/expression.js b/lib/common/expression.js
index 2fc5988..699e581 100644
--- a/lib/common/expression.js
+++ b/lib/common/expression.js
@@ -14,7 +14,7 @@ function reduce(exp, callback) {
   while (loop) {
     loop = false;
     for (let i = 2; i < exp.length; ++i) {
-      const res = callback(exp[i - 1], exp[i]);
+      const res = callback(exp[i - 1], exp[i], i - 2);
       if (res !== REDUCE_SKIP) {
         loop = true;
         exp = exp.slice();
@@ -126,10 +126,10 @@ function evaluate(exp, obj, now, cb) {
         return REDUCE_SKIP;
       });
     } else if (e[0] === "/") {
-      return reduce(e, (a, b) => {
+      return reduce(e, (a, b, i) => {
         if (!isArray(a) && !isArray(b)) {
           if (a == null || b == null) return null;
-          return a / b;
+          return i === 0 ? a / b : a * b;
         }
         return REDUCE_SKIP;
       });
@@ -142,10 +142,10 @@ function evaluate(exp, obj, now, cb) {
         return REDUCE_SKIP;
       });
     } else if (e[0] === "-") {
-      return reduce(e, (a, b) => {
+      return reduce(e, (a, b, i) => {
         if (!isArray(a) && !isArray(b)) {
           if (a == null || b == null) return null;
-          return a - b;
+          return i === 0 ? a - b : a + b;
         }
         return REDUCE_SKIP;
       });