In 2013–14 West Ham United F.C. season, West Ham finished 13th in the Premier League. They also reached the semi-finals of the Football League Cup before losing 9–0 on aggregate to eventual cup-winners Manchester City F.C.. A feature of the season were the criticisms of manager Sam Allardyce by supporters relating to his perceived negative playing tactics. West Ham finished 12th in the 2014–15 Premier League, one place higher than the previous season. Minutes after the last game of the season, on 24 May 2015, the club announced that Allardyces contract would not be renewed and that they were seeking a new manager. By winning the Premier League Fair Play league for 2014–15, West Ham qualified for the 2015–16 UEFA Europa League, entering at the first qualifying round. On 9 June 2015, former West Ham player Slaven Bilić was appointed as manager on a three-year contract. In Bilićs fourth game in charge, the team won at Anfield for the first time in 52 years, beating Liverpool 0–3, with goals from Manuel Lanzini, Mark Noble and Diafra Sakho. In Bilićs first season as manager, West Ham finished seventh in the Premier League. The team broke several records for the club in the Premier League era, including the highest number of points (62), the highest number of goals in a season (65), the least number of games lost in a season (8) and the lowest number of away defeats (5). Following Manchester Uniteds win in the 2016 FA Cup Final, West Ham took their Europa League place and qualified for the third qualifying round of the 2016–17 UEFA Europa League. In August 2016, André Ayew became the clubs most expensive signing after joining for a fee of £20.5 million from Swansea City A.F.C.. At the end of the 2016–17 Premier League season West Ham finished 11th. In July 2017, Marko Arnautović became West Hams record signing, for a fee of up to £25 million from Stoke City F.C.. In June 2018, Issa Diop (footballer) became their record signing.
Question: How many more million did Marko Arnautović sign for in July 2017 compared to André Ayew in August 2016?
Answer: 4.5