program razine;

var
   n, i, rj, razl : longint;
   bod : array[1 .. 100] of longint;

begin
   readln (n);
   for i := 1 to n do readln (bod[i]);
   for i := n-1 downto 1 do
      if bod[i]>=bod[i+1] then begin
         razl := bod[i]-bod[i+1]+1;
         bod[i] := bod[i]-razl;
         rj := rj+razl;
      end;
   writeln (rj);
end.