Effect of Smart Phone App-based Human Coaching Program in Gastrectomized Patients

"1. experimental arm~ * 1:1 coaching using smart phone app~ * Patients are provided several articles about postoperative care (symptom management, diet, and exercise) in the app.~ * Patients are encouraged to report their diet diaries and exercise everyday by the coach.~ * Patients can ask to the coach using the app and the coach answer when they check them.~2. Comparator arm - Patient have consulting with clinical nutritionist at 1 month and 3 months postoperatively."