Limbaugh holds an annual fundraising telethon called the "EIB Cure-a-Thon" for the Leukemia & Lymphoma Society. In 2006, the EIB Cure-a-Thon conducted its 16th annual telethon, raising $1.7 million, totaling over $15 million since the first cure-a-thon. According to Leukemia and Lymphoma Society annual reports, Limbaugh personally contributed between $100,000 and $499,999 from 2000–2005 and 2007, and Limbaugh said that he contributed around $250,000 in 2003, 2004 and 2005. NewsMax reported Limbaugh donated $250,000 in 2006, and the Societys 2006 annual report placed him in the $500,000 to $999,999 category. Limbaugh donated $320,000 during the 2007 Cure-a-Thon, which the Leukemia and Lymphoma Society reported had raised $3.1 million. On his radio program April 18, 2008, Limbaugh pledged $400,000 to the Leukemia and Lymphoma Society after being challenged by two listeners to increase his initial pledge of $300,000.

Answer this question based on the article: How many dollars did Limbaugh increase  his pledge by after being challenged by listeners?
100000