Open warfare broke out in 1352, when John V, supported by Venetian and Serbian troops, launched an attack on Matthew Kantakouzenos. John Kantakouzenos came to his son's aid with 10,000 Ottoman troops who retook the cities of Thrace, liberally plundering them in the process. In October 1352, at Demotika, the Ottoman force met and defeated 4,000 Serbs provided to John V by Dushan. This was the Ottomans' first victory in Europe and an ominous portent. Two years later their capture of Gallipoli marked the beginning of the Ottoman conquest of the Balkans, culminating a century later in the Fall of Constantinople. Meanwhile, John V fled to the island of Tenedos, from where he made an unsuccessful attempt to seize Constantinople in March 1353. John VI Kantakouzenos responded by having Matthew crowned as co-emperor, but John V Palaiologos, enlisting Genoese support and relying on the declining popularity of Kantakouzenos, succeeded in entering the capital in November 1354. John VI Kantakouzenos abdicated and retired to a monastery. Matthew held out in Thrace making war upon the Serbs in 1356. Then Matthew gathered an army of 5,000 Turks and marched on Serres the Serbian held capital of John Ugleisha. Stephen Urosh V whose mother also ruled at Serres decided to raise an army to defend his mother and in 1357 when Matthew and his Turks attacked, The Serbian army under Vojin The Count of Drama came to the rescue and the Turks were defeated and Matthew captured and held hostage until his ransom was paid by the Emperor John V Palaiologos who was now the sole master of a rump state. Matthew was allowed to go to the Morea and reign there with his brother Manuel.
Question: What happened second, the retaking of the cities of Thrace or the attempt to seize Constantinople?
Answer: attempt to seize Constantinople