EU leaders attend the European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 16, 2016. REUTERS/Yves Herman

European Council President Donald Tusk (3rdL), European Commission President Jean-Claude Juncker (2ndR) and European Parliament President Martin Schulz (L) pose for a group photo with Slovakia's Prime Minister Robert Fico (C), Slovakia's President Andrej Kiska (3rdR) and Speaker of the Slovakia's National Council Andrej Danko (2ndL) at the Government office ahead of the upcoming European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 15, 2016. REUTERS/Yves Herman

European Union countries flags are seen at the Bratislava Castle (Hrad) during the European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 16, 2016. REUTERS/Yves Herman

European Council President Donald Tusk (3rdL), European Commission President Jean-Claude Juncker (2ndR) and European Parliament President Martin Schulz (L) pose for a group photo with Slovakia's Prime Minister Robert Fico (C), Slovakia's President Andrej Kiska (3rdR) and Speaker of the Slovakia's National Council Andrej Danko (2ndL) at the Government office ahead of the upcoming European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 15, 2016. REUTERS/Yves Herman

European Council President Donald Tusk (3rdL), European Commission President Jean-Claude Juncker (2ndR) and European Parliament President Martin Schulz (L) pose for a group photo with Slovakia's Prime Minister Robert Fico (C), Slovakia's President Andrej Kiska (3rdR) and Speaker of the Slovakia's National Council Andrej Danko (2ndL) at the Government office ahead of an upcoming European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 15, 2016. REUTERS/Yves Herman

European Council President Donald Tusk (R) is welcomed by Slovakia's Prime Minister Robert Fico at the Government office ahead of an upcoming European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 15, 2016. REUTERS/Yves Herman

European Council President Donald Tusk arrives at the Government office ahead of the upcoming European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 15, 2016. REUTERS/Yves Herman

(L-R) Greece's Prime Minister Alexis Tsipras, Netherlands' Prime Minister Mark Rutte, France's President Francois Hollande and European Commission President Jean Claude Juncker attend the European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 16, 2016. REUTERS/Yves Herman