uses math;
const
 xx : array [1..4] of longint = (1,-1,0,0);
 yy : array [1..4] of longint = (0,0,1,-1);

var
 m:array [1..1000,1..1000] of longint;
 a:array [1..1000,1..1000] of longint;
 M1,N,i,j:longint;
 Ans:longint;
 Beg,Fin,e:longint;
 st1,st2:array [1..10000000] of longint;

Procedure BFS(q,w:longint);
begin
 Beg:=1;
 Fin:=1;
  st1[1]:=q;
  st2[1]:=w;
 m[q][w]:=1;
 While Fin>=Beg do begin
   For e:=1 to 4 do begin
     if st1[Beg]+xx[e]<=N then
     if st1[Beg]+xx[e]>=1 then
     if st2[Beg]+yy[e]<=M1 then
     if st2[Beg]+yy[e]>=1 then
        if a[st1[Beg]][st2[Beg]] + 1 = a[st1[Beg]+xx[e]][st2[Beg]+yy[e]] then
          if m[st1[Beg]][st2[Beg]] + 1 > m[st1[Beg]+xx[e]][st2[Beg]+yy[e]] then begin
                Inc(Fin);
                st1[Fin]:=st1[Beg]+xx[e];
                st2[Fin]:=st2[Beg]+yy[e];
                m[st1[Fin]][st2[Fin]]:=m[st1[Beg]][st2[Beg]]+1;
                                                                           end;
                    end;
  Inc(Beg);
                   end;
end;

begin
 Assign(input,'sherlocked.in');
 Reset(input);
 Assign(output,'sherlocked.out');
 Rewrite(output);
  Read(N,M1);
   For i:=1 to N do
    For j:=1 to M1 do Read(a[i][j]);
 For i:=1 to N do
  For j:=1 to M1 do
    if m[i][j]=0 then BFS(i,j);
 For i:=1 to N do
  For j:=1 to M1 do
    Ans:=max(Ans,m[i][j]);
 Write(Ans);
end.