Too many types conversions in a Z80 emulator in Ada

I am doing a Z80 emulator in Ada. I am implementing the JR (Jump relative) family, But I am not satisfied with my code:

with Ada.Text_IO;

procedure main is
    type UInt16 is mod 2 ** 16;
    type UInt8  is mod 2 **  8;
    type Int8   is range -128 .. 127;

    package UInt16_IO is new Ada.Text_IO.Modular_IO (UInt16);

    function Two_Complement(N : UInt8) return Int8 is
    begin
        if N <= 127 then
            return Int8 (N);
        end if;
        return Int8 (Integer (N) - 256);
    end Two_Complement;

    -- Relative jump
    function Jr (Address : UInt16; D: UInt8) return UInt16 is
    begin
        return UInt16 (Integer (Address) + Integer (Two_Complement (D) + 2));
    end Jr;

    Address : UInt16;
begin
    Address := 16#683#;
    UInt16_IO.Put (Item => Jr (Address, 16#F1#), Base => 16); -- Get    16#676# which is good !
end main;

It seems to work, but I find that there are too many types conversions. Do you have some advice ?

Thanks,

Olivier.