Program CaM(input,output);
Var
    a:array[1..1005]of longint;
    f:array[0..1005,0..1005]of longint;
    i,j,n,ans:longint;

Begin
    assign(input,'seqgame.in'); reset(input);
    assign(output,'seqgame.out'); rewrite(output);

    readln(n); ans:=0;
    for i:=1 to n do
        read(a[i]);

    for i:=1 to n do
        if a[i]=i then f[i,0]:=f[i-1,0]+1 else f[i,0]:=f[i-1,0];

    for i:=1 to n do
        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-1,j-1]>f[i,j] then f[i,j]:=f[i-1,j-1];
        End;

    for i:=0 to n do
        if f[n,i]>ans then ans:=f[n,i];
    writeln(ans);

    close(input); close(output);
End.