#define _CRT_SECURE_NO_WARNINGS
#include <cstdio>
#include <cstdlib>
#include <string>
#include <iostream>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <cmath>
#include <queue>
#include <stack>
#include <deque>
#include <cassert>
#include <ctime>
#include <climits>
using namespace std;

#ifdef WIN32
#define LLD "%I64d"
#else
#define LLD "%lld"
#endif

typedef long long ll;
typedef long double ld;

#define mp make_pair
#define pb push_back
#define sz(x) (int)(x).size()
#define X first
#define Y second
#define all(x) x.begin(),x.end()
#define y1 y11111

const int maxn = 1000 + 10;
int a[maxn][maxn], dp[maxn][maxn];
bool used[maxn][maxn];
vector< pair< int, pair<int, int> > > v;
pair<int, int> turn[4] = { mp(0, 1), mp(1, 0), mp(-1, 0), mp(0, -1) };

int n, m;
bool correct(int x, int y) {
	return (x >= 0 && y >= 0 && x < n && y < m);
}

void dfs(int x, int y) {
	used[x][y] = 1;
	for (int i = 0; i < 4; i++) {
		int newx = x + turn[i].X;
		int newy = y + turn[i].Y;
		if (correct(newx, newy) && a[x][y] + 1 == a[newx][newy]) {
			dp[newx][newy] = max(dp[newx][newy], dp[x][y] + 1);
			if (!used[newx][newy])
				dfs(newx, newy);
		}
	}
}

int main() {
#ifdef _DEBUG
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#else
	freopen("sherlocked.in", "r", stdin);
	freopen("sherlocked.out", "w", stdout);
#endif

	scanf("%d%d", &n, &m);
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < m; j++) {
			scanf("%d", &a[i][j]);
			v.pb(mp(a[i][j], mp(i, j)));
		}
	}

	sort(all(v));
	//reverse(all(v));

	for (int i = 0; i < sz(v); i++) {
		if (!used[v[i].Y.X][v[i].Y.Y]) {
			dfs(v[i].Y.X, v[i].Y.Y);
		}
	}

	int ans = 0;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < m; j++) {
			ans = max(ans, dp[i][j]);
		}
	}

	printf("%d", ans + 1);

	return 0;
}