CREATE TABLE table_204_851 (id DECIMAL, "serial number" DECIMAL, "river" TEXT, "name" TEXT, "area  ( km2 ) " DECIMAL, "district" TEXT, "co-ordinate" TEXT, "altitude  ( m ) " DECIMAL)

what is the difference in altitude between thunakkadavu and peechi ?
SELECT ABS((SELECT "altitude (m)" FROM table_204_851 WHERE "name" = 'thunakkadavu') - (SELECT "altitude (m)" FROM table_204_851 WHERE "name" = 'peechi'))
SELECT ABS((SELECT "altitude (m)" FROM "table_204_851" WHERE "name" = 'thunakkadavu') - (SELECT "altitude (m)" FROM "table_204_851" WHERE "name" = 'peechi'))