program razine;
var n, i, s: integer;
    t : array[1..100] of integer;
    o : longint;

begin
    readln(n);
    o := 0; s := 1;
    for i := 1 to n do begin
        readln(t[i]);
        if t[i] <= t[s] then
            s := i;
    end;
    for i := s-1 downto 1 do begin
        o := o + (t[i]-t[i+1]) + 1;
        t[i] := t[i] - ((t[i]-t[i+1]) + 1);
    end;
    for i := n-1 downto s+1 do begin
        o := o + (t[i]-t[i+1]) + 1;
        t[i] := t[i] - ((t[i]-t[i+1]) + 1);
    end;
    writeln(o);
end.