"""

Fraises dans une boîte(6)
想定解

3乗計算量Ver

色々と整っていないので、あまりに読み辛いです
rinさんのを読んでください…

"""

import sys
from sys import stdin

H,W = map(int,stdin.readline().split())

S = [list(map(int,stdin.readline().split())) for i in range(H)]

# 制約チェック
#assert 1 <= H <= 22
#assert 1 <= W <= 22
assert len(S) == H
for s in S:
    assert len(s) == W
    for i in s:
        assert i == 0 or i == 1

# 領域の和を高速に導出するための2次元累積和
SSum = [[0] * (W+1) for i in range(H+1)]
for i in range(H):
    for j in range(W):
        SSum[i+1][j+1] += S[i][j]
for i in range(H):
    for j in range(W+1):
        SSum[i+1][j] += SSum[i][j]
for i in range(H+1):
    for j in range(W):
        SSum[i][j+1] += SSum[i][j]

# 同じ行で、i番目からみて直左にあるjのインデックスを記録
SR = [[-1] * W for i in range(H)]
for i in range(H):
    for j in range(W):
        if S[i][j] == 1:
            SR[i][j] = j
for i in range(H):
    for j in range(W-1):
        SR[i][j+1] = max(SR[i][j+1] , SR[i][j])

# メモ化用の2次元list
inf = float("inf")
lis = [[inf] * (W+1) for i in range(H+1)]
for j in range(W+1):
    lis[H][j] = 0
for i in range(H+1):
    lis[i][W] = 0

# ここからdp

for x1 in range(H-1,-1,-1): #左上のx座標
    for y1 in range(W-1,-1,-1): #左上のy座標

        if SSum[H][W] - SSum[x1][W] - SSum[H][y1] + SSum[x1][y1] <= 1:
            lis[x1][y1] = 0
            continue

        # ここから、最小値の導出処理
        now_ans = inf

        # 左上領域の空行カウント用の配列
        zerox = [0]
        zeroy = [0]

        for tmpx in range(x1,H):
            nsum = SSum[tmpx+1][W] - SSum[tmpx][W] - SSum[tmpx+1][y1] + SSum[tmpx][y1]
            zerox.append(zerox[-1] + (min(1,nsum)^1) )

        for tmpy in range(y1,W):
            nsum = SSum[H][tmpy+1] - SSum[H][tmpy] - SSum[x1][tmpy+1] + SSum[x1][tmpy]
            zeroy.append(zeroy[-1] + (min(1,nsum)^1) )
        
        for xmid in range(H,x1-1,-1):

            # ymidは一意に決まる
            if xmid == H:
                ymids = [i for i in range(y1,W+1)]
            elif xmid == H-1:
                ymids = [max(y1,SR[xmid][-1]+1)]
            else:
                ymids = [max(ymids[0] , SR[xmid][-1]+1)]

            for ymid in ymids:

                assert x1 <= xmid <= H
                assert y1 <= ymid <= W
                
                #右下が0で無い場合、駄目
                if SSum[H][W] - SSum[xmid][W] - SSum[H][ymid] + SSum[xmid][ymid] != 0:
                    continue
                #左上・右下共に0の場合は推移の意味が無いのでダメ
                if (xmid-x1) * (ymid-y1) == 0 and (H-xmid) * (W-ymid) == 0:
                    continue

                cost = 0

                # 左上区間の1の個数
                lu1 = SSum[xmid][ymid] - SSum[xmid][y1] - SSum[x1][ymid] + SSum[x1][y1]
                cost += (xmid-x1-zerox[xmid-x1]) * (ymid-y1-zeroy[ymid-y1]) - lu1

                # 左下と右上
                cost += lis[xmid][y1] + lis[x1][ymid]

                # デバッグ用
                #print (x1,y1,xmid,ymid,"cost",cost,lu1)

                if xmid == x1 and ymid == y1:
                    continue
                now_ans = min(now_ans , cost)

          
        lis[x1][y1] = now_ans

print (lis[0][0])