program razine;
var a:array[1..100] of integer;
    i,n:integer;
    r:longint;
begin
 readln(n);
 r:=0;
 for i:=1 to n do readln(a[i]);
 for i:=n downto 1 do begin
                    if a[i-1]>=a[i] then begin
                                          r:=a[i-1]-(a[i]-1)+r;
                                          a[i-1]:=a[i]-1;
                                         end;
                   end;
 writeln(r);

end.