var a:array[1..10000] of longint;
    n,i,j,max,x:longint;
begin
read(n);
max:=-100;
for i:=1 to n do
read(a[i]);
for i:=1 to n do
 if a[i]+1=a[i+1] then
   begin
   x:=1;
   for j:=i to n do
   begin
   if a[j]+1=a[j+1] then x:=x+1 else break;
   if max<x then max:=x;
   end;
   end;
write(max);
end.