The small Diocese of Oulu has only five parishes, the largest of which is Oulu. Traditionally, the Skolts, now a small minority of only 400 speakers, have been the earliest Orthodox Christians in the Finnish Lapland. Today, they live predominantly in the Inari parish. The diocese was established in 1980. It has fewer than 10,000 members. The cathedral of the diocese is the Holy Trinity Cathedral of Oulu. The head of the diocese since 1997 has been Metropolitan Panteleimon.The head of the diocese since 2015 is Metropolitan Elia.
Question: How many years after Metropolitan Panteleimon became head of the diocese did Metropolitan Elia become the head?
Answer: 18