### BUGGY CODE ###

import sys, bisect, math, itertools, string, queue, copy
# import numpy as np
# import scipy
from collections import Counter,defaultdict,deque
from itertools import permutations, combinations
from heapq import heappop, heappush
# input = sys.stdin.readline
sys.setrecursionlimit(10**8)
mod = 10**9+7
def inp(): return int(input())
def inpm(): return map(int,input().split())
def inpl(): return list(map(int, input().split()))
def inpls(): return list(input().split())
def inplm(n): return list(int(input()) for _ in range(n))
def inplL(n): return [list(input()) for _ in range(n)]
def inplT(n): return [tuple(input()) for _ in range(n)]
def inpll(n): return [list(map(int, input().split())) for _ in range(n)]
def inplls(n): return sorted([list(map(int, input().split())) for _ in range(n)])

def main():
    h,w = map(int,input().split())
    c = [tuple(input()) for _ in range(h)]
    ans = 0
    for j in range(w-1):
        w = [[0 for _ in range(h+1)] for _ in range(h+1)]
        
        for x in range(h+1):
            for z in range(h-x):
                if c[x+z][j] == c[z][j+1]:
                    w[0][x] += 1
        for x in range(1,h+1):
            for z in range(h-x):
                if c[z][j] == c[x+z][j+1]:
                    w[x][0] += 1
        for x in range(h+1):
            for y in range(h+1):
                if x == 0 or y == 0:
                    continue
                else:
                    if c[h-x][j] == c[h-y][j+1]:
                        w[x][y] = w[x-1][y-1]-1
                    else:
                        w[x][y] = w[x-1][y-1]
        dp = [[0 for _ in range(h+1)] for _ in range(h+1)]
        dp[0][0] = 0
        for x in range(h+1):
            for y in range(h+1):
                if x == 0 and y == 0:
                    continue
                if x == 0:
                    dp[x][y] = dp[x][y-1]+w[x][y-1]
                elif y == 0:
                    dp[x][y] = dp[x-1][y]+w[x-1][y]
                else:
                    dp[x][y] = min(dp[x][y-1]+w[x][y-1],dp[x-1][y]+w[x-1][y])
        ans += dp[h][h]
    print(ans)
    
if __name__ == "__main__":
    main()

### FIXED CODE ###

import sys
input = sys.stdin.readline
def main():
    h,W = map(int,input().split())
    c = [tuple(input()) for _ in range(h)]
    c = tuple(c)
    ans = 0
    w = [0 for _ in range((h+1)**2)]
    dp = [0 for _ in range((h+1)**2)]
    for j in range(W-1):
        for x in range(h,0,-1):
            for y in range(h,0,-1):
                w[(x-1)*(h+1)+y-1] = w[x*(h+1)+y]
                if c[h-x][j] == c[h-y][j+1]:
                    w[(x-1)*(h+1)+y-1] += 1
        for x in range(h+1):
            for y in range(h+1):
                if x == 0 or y == 0:
                    if x == y:
                        dp[x*h+y] = 0
                    elif x == 0:
                        dp[x*(h+1)+y] = dp[x*(h+1)+y-1]+w[x*(h+1)+y-1]
                    else:
                        dp[x*(h+1)+y] = dp[(x-1)*(h+1)+y]+w[(x-1)*(h+1)+y]
                    continue
                dp[x*(h+1)+y] = min(dp[x*(h+1)+y-1]+w[x*(h+1)+y-1],dp[(x-1)*(h+1)+y]+w[(x-1)*(h+1)+y])
        ans += dp[-1]
    print(ans)
    
if __name__ == "__main__":
    main()