Edited by author 15.11.2009 18:42

program Project286286;

{$APPTYPE CONSOLE}

uses
  SysUtils,Math;



var a,b,c:array[0..2000000]of longint;
i,j,m,n,f:longint;

function RMQ(l,r:longint):longint;
var res,i,j:longint;
begin
 i := l + n - 1;
 j := r + n - 1;
 res:=0;
 while i<=j do
   begin
     res := max(res,max(a[i],a[j]));
     i:=(i+1) div 2;
     j:=(j-1) div 2
   end;
 result:=res;
end;


begin
 reset(input,'input.txt');
 rewrite(output,'output.txt');
 readln(n);
 for i := 1 to n do
   readln(c[i]);
 for i := 1 to n do
   c[i]:=c[i]-1+c[i-1];
 for i := 1 to n do
   b[i]:=c[i]-c[i-1];
 for i := n to 2*n-1 do
   a[i]:=b[i-n+1];
 for i := n-1 downto 1 do
   a[i]:= max(a[i*2],a[i*2+1]);
 for i := 1 to n - 1 do
   if rmq(i+1,n)-c[i-1]>n then
     begin
       writeln('NO');
       halt(0);
     end;
 writeln('YES');
end.

why wa???