Const
    ProgramName='seqgame';
Var
    i,j,k,n,m,l,r,s,t,x,y,ans:longint;
    a:array[0..1100]of longint;
    f:array[0..1100,0..1100]of longint;
 
Procedure Pin;
var
    i,j,k:longint;
begin
    readln(n);
    for i:=1 to n do
        read(a[i]);
    readln;
end;
 
 
Procedure Main;
var
    i,j,k:longint;
begin
    fillchar(f,sizeof(f),0);
    ans:=0;
    for i:=1 to n do
        begin
            if a[i]=i then
                f[i,0]:=f[i-1,0]+1
            else
                f[i,0]:=f[i-1,0];
            for j:=1 to i-1 do
                begin
                    if a[i]=i-j then
                        f[i,j]:=f[i-1,j]+1
                    else
                        f[i,j]:=f[i-1,j];
                    if f[i,j]<f [i-1,j-1] then
                        f[i,j]:=f[i-1,j-1];
                    if i=n then
                        if ans<f[i,j] then
                            ans:=f[i,j];
            end;
        end;
end;
 
 
Procedure Pout;
var
    i,j,k:longint;
begin
    writeln(ans);
end;
 
 
begin
    assign(input,ProgramName+'.in');
    assign(output,ProgramName+'.out');
    reset(input);
    rewrite(output);
    pin;
    main;
    pout;
    close(input);
    close(output);
end.