program operatii;
type vector=array[1..1000000] of longint;
var f,g:text;v:vector;n,op,i:longint;

procedure operatie;
var k,k2,k3:longint;ok,ok2:boolean;
begin
k:=0;
k2:=0;
repeat
k2:=k2+1;
ok2:=false;
for k:=1 to n+1 do
begin
if v[k]>=k2 then
   begin
   ok:=true;
   ok2:=true;
   end;
if ok=true then
   if (v[k]<k2) and (v[k-1]>=k2) then
      begin
      op:=op+1;
      end;
end;
until ok2=false;
end;

begin
assign(f,'operatii.in');
reset(f);
readln(f,n);
for i:=1 to n do
    read(f,v[i]);
close(f);
operatie;
assign(g,'operatii.out');
rewrite(g);
write(g,op);
close(g);
end.