A pilot looks up from a U.S. F-22 Raptor fighter as it prepares to refuel in mid-air with a KC-135 refuelling plane over European airspace during a flight to Britain from Mihail Kogalniceanu air base in Romania April 25, 2016. (Photo by Toby Melville/Reuters)

A woman sits on a terrace at Tiki hostel in Cantagalo favela, in Rio de Janeiro, Brazil, April 16, 2016. Hostels in a few of Rio's more than 1,000 slums serve not only as a cheap housing alternative for the more adventurous among the estimated 500,000 foreign tourists expected to arrive for the Olympics in August. The establishments also open up the rich culture of the city's shantytowns for travellers, giving them a glimpse into once “no-go” areas where about one-fifth of Rio's population lives. (Photo by Pilar Olivares/Reuters)

Elderly people attend an aquatic exercise at a private pool in Bangkok, Thailand, April 28, 2016. Many Thai families look after elderly relatives at a cost that countrywide adds up to just under a third of household income. The number of families facing this issue will balloon as the population ages at a rate among the fastest in Southeast Asia. (Photo by Athit Perawongmetha/Reuters)

An Israeli boy looks through the sight of a weapon during a display of Israeli Defense Forces (IDF) equipment and abilities at the West Bank settlement of Kiryat Arba, April 23, 2015, during celebrations for Israel's Independence Day, marking the 67th anniversary of the creation of the state. (Photo by Ronen Zvulun/Reuters)

French climber Alain Robert, also known as “Spiderman”, holds the flag of Nepal as he scales the Tour Montparnasse, a 210-metre (689 ft) building in central Paris, France April 28, 2015, to show support for the victims after the earthquake in Nepal. (Photo by Gonzalo Fuentes/Reuters)

Russian servicemen drive a partially covered T-14 Armata tank along a street before a rehearsal for the Victory Day parade in Moscow, Russia, April 29, 2015. Russia will celebrate the 70th anniversary of the victory over Nazi Germany in World War Two on May 9. (Photo by Maxim Zmeyev/Reuters)

Vietnamese military personnel in traditional dress take part in a parade celebrating the 40th anniversary of the end of the Vietnam War which is also remembered as the fall of Saigon, in Ho Chi Minh City, Vietnam, Thursday, April 30, 2015. (Photo by Dita Alangkara/AP Photo)

An owner poses with her Maine Coon cat in front of a mirror during a cat exhibition in Bishkek on April 19, 2015. Cat lovers from Kyrgyzstan, Kazakhstan and Uzbekistan took part in the exhibition. (Photo by Vyacheslav Oseledko/AFP Photo)