question: What is the total frequency mhz of the kgrj call sign, which has an erp w greater than 21,500?
answer: SELECT COUNT(frequency_mhz) FROM table_name_19 WHERE call_sign = "kgrj" AND erp_w > 21 OFFSET 500
context: CREATE TABLE table_name_19 (frequency_mhz VARCHAR, call_sign VARCHAR, erp_w VARCHAR)