Question: Who is the paternal grandfather of Bolesław Januszowic?

Knowledge 1: Bolesław Januszowic
Bolesław Januszowic (1385/86 – ca. 4 May 1424), was a Polish prince member of the House of Piast.He was the second son of Duke Janusz I of Warsaw and Danutė of Lithuania, a daughter of Kęstutis. [SEP] Knowledge 2: Janusz I of Warsaw
Janusz I of Warsaw (pl: "JanuszI warszawski"), also known as Janusz I the Old(pl: "Janusz I Starszy")(c. 1347/52 – 8 December 1429), was a Polish prince member of the House of Piast in the Masovian branch, from 1373/74 Duke of Warsaw and after the division of the paternal inheritance between him and his brother in 1381, ruler over Nur, Łomża, Liw, Ciechanów, Wyszogród and Zakroczym.In addition, he was a vassal of the Polish Kingdom since 1391 for the fief of Podlachia (only during his lifetime).He was the eldest son of Siemowit III, Duke of Masovia and his first wife Euphemia, daughter of Nicholas II of Opava.Due to an error of chronicler Jan Długosz was previously assumed that Janusz I was born c. 1329, and it wasn't until modern time that this date could be corrected until a much later one, c. 1346.Evidence of this fact was that only in 1373/74 he received his own duchy (with its capital in Warsaw).As the result of the partition of Masovia between him and his younger brother Siemowit IV after the death of their father on 16 June 1381, Janusz I finally obtain the totally of his domains: Warsaw, Nur, Łomża, Liw, Ciechanów, Wyszogród and Zakroczym.
Knowledge 1: Janusz I of Warsaw
Janusz I of Warsaw (pl: "JanuszI warszawski"), also known as Janusz I the Old(pl: "Janusz I Starszy")(c. 1347/52 – 8 December 1429), was a Polish prince member of the House of Piast in the Masovian branch, from 1373/74 Duke of Warsaw and after the division of the paternal inheritance between him and his brother in 1381, ruler over Nur, Łomża, Liw, Ciechanów, Wyszogród and Zakroczym.In addition, he was a vassal of the Polish Kingdom since 1391 for the fief of Podlachia (only during his lifetime).He was the eldest son of Siemowit III, Duke of Masovia and his first wife Euphemia, daughter of Nicholas II of Opava.Due to an error of chronicler Jan Długosz was previously assumed that Janusz I was born c. 1329, and it wasn't until modern time that this date could be corrected until a much later one, c. 1346.Evidence of this fact was that only in 1373/74 he received his own duchy (with its capital in Warsaw).As the result of the partition of Masovia between him and his younger brother Siemowit IV after the death of their father on 16 June 1381, Janusz I finally obtain the totally of his domains: Warsaw, Nur, Łomża, Liw, Ciechanów, Wyszogród and Zakroczym. [SEP] Knowledge 2: Bolesław IV of Warsaw
Bolesław IV of Warsaw( also known as Boleslaus IV of Warsaw,; – 10 September 1454), was a Polish prince member of the House of Piast in the Masovian branch.He was Duke of Warsaw during 1429 – 1454( under regency until 1436) and sovereign Duke of Podlachia in 1440 – 1444.He was the second son of Bolesław Januszowic( in turn second son of Janusz I of Warsaw) and Anna Feodorovna of Ratnie, a Lithuanian princess( granddaughter of Algirdas).
Knowledge 1: Bolesław Januszowic
Bolesław Januszowic (1385/86 – ca. 4 May 1424), was a Polish prince member of the House of Piast.He was the second son of Duke Janusz I of Warsaw and Danutė of Lithuania, a daughter of Kęstutis. [SEP] Knowledge 2: Bolesław IV of Warsaw
Bolesław IV of Warsaw( also known as Boleslaus IV of Warsaw,; – 10 September 1454), was a Polish prince member of the House of Piast in the Masovian branch.He was Duke of Warsaw during 1429 – 1454( under regency until 1436) and sovereign Duke of Podlachia in 1440 – 1444.He was the second son of Bolesław Januszowic( in turn second son of Janusz I of Warsaw) and Anna Feodorovna of Ratnie, a Lithuanian princess( granddaughter of Algirdas).

Answer: Siemowit III, Duke of Masovia