program Untitled;
var
   n, i, rj : longint;
   a : array [ 0..105 ] of longint;
begin
     readln ( n );
     for i := 1 to n do
     begin
          readln ( a[i] );
     end;
     for i := n - 1 downto 1 do
     begin
          if a[i] >= a[i + 1] then
          begin
               rj := rj + (a[i] - a[i + 1]) + 1;
               a[i] := a[i + 1] - 1;
          end;
     end;
     writeln ( rj );
readln;
end.