Who was the President of Czechoslovakia when an ambassador's tenure began in 1950?
CREATE TABLE "ambassadors_to_czechoslovakia" (
  "name" text,
  "tenure_begin" text,
  "tenure_end" real,
  "british_monarch" text,
  "czechoslovak_president" text
);
SELECT "czechoslovak_president" FROM "ambassadors_to_czechoslovakia" WHERE "tenure_begin"='1950';