import sys
r, s = map(int, sys.stdin.readline().split())

N = 25
g = [[" "] * N for i in range(N)]

for i in range(r):
    row = input()
    for j in range(s):
        g[i][j] = row[j]

dx = [-1, 0, 1, 0]
dy = [0, 1, 0, -1]
st = [False] * 30
ans = 0
def dfs(x, y, count):
    global ans
    ans = max(ans, count)

    for i in range(4):
        a = x + dx[i]
        b = y + dy[i]
        t = ord(g[a][b]) - ord('A')
        if 0 <= a < r and 0 <= b < s and not st[t]:
            st[t] = True
            dfs(a, b, count + 1)
            st[t] = False

st[ord(g[0][0]) - ord('A')] = True
dfs(0, 0, 1)
print(ans)