h,w = map(int,input().split())
s = [input() for i in range(h)]
now = 0
l = set([(0,0)])
ans = s[0][0]
while now < h+w-1:
  new = set()
  mn = "z"
  if (h-1, w-1) in l: break
  for i,j in l:
    if i == h-1:
      mn = min(mn, s[i][j+1])
    elif j == w-1:
      mn = min(mn, s[i+1][j])
    else:
      mn = min(mn, s[i][j+1], s[i+1][j])
  for i,j in l:
    if i == h-1:
      if s[i][j+1] == mn:
        new.add((i,j+1))
    elif j == w-1:
      if s[i+1][j] == mn:
        new.add((i+1,j))
    else:
      if s[i][j+1] == mn:
        new.add((i,j+1))
      if s[i+1][j] == mn:
        new.add((i+1,j))
  l = new
  now += 1
  ans += mn
print(ans)