Complex sentence: Melgar won the Campeonato Descentralizado for the first time in 1981 and again in 1983, but that year the top six teams played an playoff tournament to determine the year's champion and Melgar finished second.
Split into simpler sentences: Melgar won the Campeonato Descentralizado for the first time in 1981 and again in 1983. In that latter year, the top six teams played a playoff tournament to determine the year's champion and Melgar finished second.