diff --git a/server/gameserver/room.cc b/server/gameserver/room.cc
index d144e9a..b10b2c0 100644
--- a/server/gameserver/room.cc
+++ b/server/gameserver/room.cc
@@ -464,7 +464,7 @@ void Room::UpdateGas()
                     ((frame_no - gas_data.gas_start_frameno) * (1.0 / SERVER_FRAME_RATE));
                 gas_data.gas_progress = std::max(gas_data.rad_old - distance, gas_data.rad_new);
                 if (!(gas_data.pos_new == gas_data.pos_old)) {
-                    Vector2D p1 = gas_data.pos_old_bk - gas_data.pos_new;
+                    Vector2D p1 = gas_data.pos_new - gas_data.pos_old_bk;
                     if (std::abs(p1.Norm() - distance) <= 0.01f) {
                         gas_data.pos_old = gas_data.pos_new;
                     } else {