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