SELECT sum(Num_of_Audience) FROM festival_detail
What are the total number of the audiences who visited any of the festivals?
[Schema (values) (types)]: | entertainment_awards |  festival_detail : festival_id (text) , festival_name (number) , chair_name (text) , location (text) , year (text) , num_of_audience (number) | artwork : artwork_id (text) , type (number) , name (text) | nomination : artwork_id (text) , festival_id (number) , result (text);