Question: Who is the paternal grandfather of Catherine Of Masovia?

Knowledge 1: Catherine of Masovia
Catherine of Masovia (pl: "Katarzyna mazowiecka"; 1413/16 – betw.2 June 1479/5 July 1480) was a Polish princess member of the House of Piast in the Masovian branch.She was the eighth and youngest daughter of Siemowit IV, Duke of Masovia and Alexandra, a daughter of Algirdas, Grand Duke of Lithuania and sister of King Władysław II Jagiełło of Poland. [SEP] Knowledge 2: Siemowit IV, Duke of Masovia
Siemowit IV (Ziemowit IV), also known as Siemowit IV the Younger(pl: "Siemowit IV Młodszy"; ca. 1353/1356 – 21 January 1426), was a Polish prince member of the House of Piast from the Masovian branch, from 1373/74 Duke of Rawa, and after the division of the paternal inheritance between him and his brother in 1381, ruler over Rawa, Płock, Sochaczew, Gostynin, Płońsk and Wizna, since 1386 hereditary Polish vassal, since 1388 ruler over Belz, during 1382–1401 he lost Wizna and during 1384–1399 and 1407–1411 he lost Zawkrze, during 1384–1399 he lost Płońsk, taken by the Teutonic Order.He was the second son of Siemowit III, Duke of Masovia and his first wife Euphemia, daughter of Nicholas II of Opava.Already during his father's lifetime, Siemowit IV received his own district, Rawa Mazowiecka (ca. 1373/74), and as a result of the partition of Masovia between him and his older brotherJanuszI after the death of their father on 16 June 1381, Siemowit IV finally obtain the totality of his domains:Rawa, Płock, Sochaczew, Gostynin, Płońsk and Wizna.
Knowledge 1: Catherine of Masovia
Catherine of Masovia (pl: "Katarzyna mazowiecka"; 1413/16 – betw.2 June 1479/5 July 1480) was a Polish princess member of the House of Piast in the Masovian branch.She was the eighth and youngest daughter of Siemowit IV, Duke of Masovia and Alexandra, a daughter of Algirdas, Grand Duke of Lithuania and sister of King Władysław II Jagiełło of Poland. [SEP] Knowledge 2: Siemowit I of Masovia
Siemowit I of Masovia( c. 1224/28 – 23 June 1262), was a Polish prince member of the House of Piast, Duke of Czersk during 1247- 1248, Duke of Masovia( except Dobrzy ń) during 1248- 1262, ruler over Sieradz during 1259- 1260.He was the sixth son of Konrad I of Masovia and his wife Agafia of Rus.He was named after his older brother Siemowit, who died young in 1224.
Knowledge 1: Siemowit IV, Duke of Masovia
Siemowit IV (Ziemowit IV), also known as Siemowit IV the Younger(pl: "Siemowit IV Młodszy"; ca. 1353/1356 – 21 January 1426), was a Polish prince member of the House of Piast from the Masovian branch, from 1373/74 Duke of Rawa, and after the division of the paternal inheritance between him and his brother in 1381, ruler over Rawa, Płock, Sochaczew, Gostynin, Płońsk and Wizna, since 1386 hereditary Polish vassal, since 1388 ruler over Belz, during 1382–1401 he lost Wizna and during 1384–1399 and 1407–1411 he lost Zawkrze, during 1384–1399 he lost Płońsk, taken by the Teutonic Order.He was the second son of Siemowit III, Duke of Masovia and his first wife Euphemia, daughter of Nicholas II of Opava.Already during his father's lifetime, Siemowit IV received his own district, Rawa Mazowiecka (ca. 1373/74), and as a result of the partition of Masovia between him and his older brotherJanuszI after the death of their father on 16 June 1381, Siemowit IV finally obtain the totality of his domains:Rawa, Płock, Sochaczew, Gostynin, Płońsk and Wizna. [SEP] Knowledge 2: Sophia of Masovia
Sophia of Masovia( 1497/1498- before 11 March 1543) was a Princess of Masovia, daughter of Konrad III Rudy, Duke of Masovia and his wife Anna Radziwiłł.She was a member of the House of Piast.On 17 September 1520 in Warsaw, Sophia was married by proxy to Stephen VII Báthory.On 17 January 1521, Sophia and her entourage went to Hungary, where in February there was a formal marriage.The couple had three children:Anna, Stephen and Klara; however, other sources claim that Klara was the couple's only child.Klara was engaged to Carl, Duke of Münsterberg, but died in 1535, before the marriage took place.After the death of Stephen in 1530, Sophia was married to Croatian magnate Louis Pekry.Sophia's second marriage was childless.She then died sometime before 11 March 1543.Sophia had three siblings: Stanislaus I of Masovia, Janusz III of Masovia and Anna of Masovia, both her brothers died childless and so Anna and Sophia were the last Masovian Piasts.Soon after Janusz's death, the Duchy of Masovia was annexed into the kingdom of Poland, despite resistance from some of the Masovian nobility.They tried to retain their independence and argued that the Duchy should be inherited by the female relatives( such as Anna or Sophia).The Polish king refused to recognize their demands.Anna was made titular Duchess of Masovia.

Answer: Siemowit III, Duke of Masovia