MAXR

#define task "MAXR"
#include <iostream>
#include <cstdio>

using namespace std;

using ll = long long;
using ld = long double;
template <class T>
void readInt(T &x)
{
    x = 0;
    register int c;
    while ((c = getchar()) && (c > '9' || c < '0'))
        ;
    for (; c >= '0' && c <= '9'; c = getchar())
        x = x * 10 + c - '0';
}

constexpr int N = 5e3 + 5;
int m, n;
int a[N][N], f[N][N];
int b[N * 2][N * 2];

void Read()
{
    // cin >> m >> n;
    readInt(m), readInt(n);

    for (int i = 1; i <= m; ++i)
        for (int j = 1; j <= n; ++j)
            // cin >> a[i][j];
            readInt(a[i][j]);
}

int S(int k)
{
    // 1 3 5 ... 2k + 1 ... 5 3 1
    return k * (2 * k) + 2 * k + 1;
}

int Get(int x, int y, int z, int t)
{
    return b[z][t] - b[x - 1][t] - b[z][y - 1] + b[x - 1][y - 1];
}

int Get(int i, int j, int k)
{
    int x = i + j - 1,
        y = i + n - j;

    return Get(x - k, y - k, x + k, y + k);
}

void Solve()
{
    for (int i = 1; i <= m; ++i)
        for (int j = 1; j <= n; ++j)
            b[i + j - 1][i + (n - j)] = a[i][j];

    for (int i = 1; i <= m + n - 1; ++i)
        for (int j = 1; j <= m + n - 1; ++j)
            b[i][j] += b[i - 1][j] + b[i][j - 1] - b[i - 1][j - 1];

    int ans(0);

    for (int i = 1; i <= m; ++i)
        for (int j = 1; j <= n; ++j)
        {
            f[i][j] = min(f[i - 1][j] + 1, f[i][j - 1] + 1);
            f[i][j] = min(f[i][j], min(i - 1, j - 1));
            f[i][j] = min(f[i][j], min(m - i, n - j));

            while (f[i][j] && Get(i, j, f[i][j]) != S(f[i][j]))
                --f[i][j];

            ans = max(ans, f[i][j]);
        }

    cout << ans;
}

int32_t main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    if (fopen(task ".INP", "r"))
    {
        freopen(task ".INP", "r", stdin);
        freopen(task ".OUT", "w", stdout);
    }

    Read();
    Solve();
}