Question:
Who was the head of state of Principality of Polotsk in May, 1021?

Context:
 Vseslav Izyaslavich is the head of the state of Principality of Polotsk from Jan, 1001 to Jan, 1003. 
 Izyaslav of Polotsk is the head of the state of Principality of Polotsk from Jan, 988 to Jan, 1001. 
 Bryachislav of Polotsk is the head of the state of Principality of Polotsk from Jan, 1003 to Jan, 1044.

Answer:
Bryachislav of Polotsk