calculate sum of overlapping dates

if we have these data :

   1    1/1/2018 13:00           1/1/2018 13:30

   2    1/1/2018 13:10           1/1/2018 13:45

   3    1/1/2018 13:20           1/1/2018 14:00

   4    1/1/2018 15:00           1/1/2018 15:30

if we want to calculate sum duration

   1    1/1/2018 13:00           1/1/2018 13:30     30

   2    1/1/2018 13:10           1/1/2018 13:45     35

   3    1/1/2018 13:20           1/1/2018 14:00     40

   4    1/1/2018 15:00           1/1/2018 15:30     30

so sum duration is : 135

but this result is wrong because we have overlapping date

   1    1/1/2018 13:00           1/1/2018 13:30     30

   2    1/1/2018 13:10           1/1/2018 13:45     15

   3    1/1/2018 13:20           1/1/2018 14:00     15

   4    1/1/2018 15:00           1/1/2018 15:30     30

so sum duration is : 90

this is correct result.

how i solve this issue ?