1 oz Campari1 oz dry gin½ oz lemon juice1½ oz red grapefruit juice½ oz simple syrup (1:1)1 top soda water
instructions :

In a mixing tin, combine the lemon juice, simple syrup, grapefruit juice, Campari and gin with ice and shake until chilled.
Strain into a Collins glass over fresh ice and top with soda water before garnishing.

source :