var n,i: integer;
    niz: array[1..100] of integer;
    min: longint;
begin
min:=0;
readln (n);
for i:=1 to n do
   readln (niz[i]);
for i:=n downto 2 do
   if niz[i]<=niz[i-1] then begin
                               min:=min+(niz[i-1]-niz[i])+1;
                               niz[i-1]:=niz[i]-1;
                            end;
writeln (min);

end.