var n:integer;
    sz:array[1..5000]of integer;
    lie:boolean;
procedure init;
var t,k:integer;
    c:char;
begin
     fillchar(sz,sizeof(sz),0);
     readln(n);
     for t:=1 to n do
         begin
              read(c);
              while c<>' ' do
                    read(c);
              read(k);
              if k=0 then
                 readln
                     else
                 readln(sz[t]);
         end;
end;
procedure make;
var t,k,v,l,m:integer;
begin
     lie:=false;
     v:=sz[n];
     k:=v;
     for t:=n-1 downto 1 do
         begin
              if sz[t]=0 then
                 k:=k+v
                         else
                 if sz[t]>v then
                    begin
                         k:=k+sz[t];
                         v:=sz[t];
                    end
                            else
                    begin
                         lie:=true;
                         break;
                    end;
         end;
     l:=k;
     if not lie then
        begin
             v:=sz[1];
             k:=v;
             for t:=2 to n do
                 begin
                      if sz[t]=0 then
                         inc(k,v)
                                 else
                         if sz[t]<v then
                            begin
                                 inc(k,v);
                                 v:=sz[t];
                            end
                                    else
                            begin
                                 lie:=true;
                                 break;
                            end;
                 end;
             m:=k;
             if not lie then
                begin
                     if not((l<=10000)and(m>=10000)) then
                        lie:=true;
                end;
        end;
end;
procedure print;
begin
     if lie then
        writeln('NO')
            else
        writeln('YES');
end;
begin
     init;
     make;
     print;
end.