SQL> ed
Wrote file afiedt.buf

  1  with t (d,s) as (
  2  select to_date('11.06.2017','dd.mm.yyyy'), 80 from dual union all
  3  select to_date('12.06.2017','dd.mm.yyyy'), 80 from dual union all
  4  select to_date('13.06.2017','dd.mm.yyyy'), 50 from dual union all
  5  select to_date('14.06.2017','dd.mm.yyyy'), 50 from dual union all
  6  select to_date('15.06.2017','dd.mm.yyyy'), 50 from dual union all
  7  select to_date('16.06.2017','dd.mm.yyyy'), 80 from dual union all
  8  select to_date('17.06.2017','dd.mm.yyyy'), 80 from dual union all
  9  select to_date('18.06.2017','dd.mm.yyyy'), 80 from dual union all
 10  select to_date('19.06.2017','dd.mm.yyyy'), 80 from dual union all
 11  select to_date('20.06.2017','dd.mm.yyyy'), 80 from dual)
 12  ,tt as (
 13  select t.*
 14  ,row_number() over (order by d desc)
 15  -row_number() over (partition by s order by d desc) gr
 16  from t
 17  )
 18* select max(d)-min(d)+1 delta from tt where gr=0
SQL> /

     DELTA
----------
         5