It is the piano part of the BGM "We must go" from 『風が強く吹いている 』. Please see http://kazetsuyo-anime.com/music/ for the detail of original OST.楽譜→ https://app.box.com/s/6131mpxsolyxpvgr421msg5e3z0nviafMy Youtube Channel→ https://www.youtube.com/channel/UCddtyyzHy8J3uTEgsiIgKgQ