var
        n,i,k,j:integer;
        b:array [1..100] of integer;
begin
        readln (n);
        for i:=1 to n do readln (b[i]);
        for j:=1 to 20000 do
        for i:=1 to n-1 do begin
                        if b[i]>=b[i+1] then begin while b[i]>=b[i+1] do begin
                        dec (b[i]);
                        inc (k);
                        end;end;
                end;
        writeln (k);
end.