CREATE TABLE table_204_593 (id DECIMAL, "year" TEXT, "chairperson" TEXT)

for how many consecutive years was challa narasimha naidu the chairperson ?
SELECT "year" - "year" FROM table_204_593 WHERE "chairperson" = 'challanarasimhanaidu'
SELECT "year" - "year" FROM "table_204_593" WHERE "chairperson" = 'challanarasimhanaidu'