program razine;

var a,b,c,d,i,n:integer;
    x,y:array[1..100] of integer;

begin

      readln(n);
                   b:=0;
        for i:=1 to n do readln(x[i]);



           for i:=n downto 2 do begin
                  if (x[i-1] > x[i]) or (x[i-1] = x[i]) then begin
                  a:=x[i-1];
                  x[i-1]:=x[i]-1;
                  b:=b + a - x[i-1];
                  end;

                   end;
                   writeln(abs(b));

end.