Within the Tašmajdan park a sports complex of Tašmajdan Sports Centre is located. Centre administers several facilities located outside Tašmajdan, like "Pionir Hall" and "Ice Hall". However, swimming pools are located in the park. The outdoor swimming pool was built in 1959-1961. Its dimensions are 50 x 20 meters; its depth varies between 2.2 and 5.0 meters; its capacity is 3,500 m³ and 2,500 seats. Next to the big one, there is a small swimming pool for children. Altogether, there is enough room for 4,000 people. It is equipped for international day-and-night competitions in swimming, water polo, water diving, etc. and also used for certain cultural venues or as an outdoor cinema during summer. It was one of the venues for the 2006 Mens European Water Polo Championship and one of the venues of the 2009 Summer Universiade in July 2009, the event for which the pool was renovated. The indoor swimming-pool was built in 1964-1968. Its dimensions are 50 x 20 meters and depth is between 2.2 and 5.4 meters; its capacity is 3,700 m³. The swimming pool is surrounded with four diving boards - 1, 3, 5 and 10 meters high and 2,000 seats. It is equipped with underwater light. At  -16 °C, water and air can be heated up to 28 °C.
Question: How many years did it take to build the indoor swimming pool?
Answer: 4