Fire Net from http://acm.zju.edu.cn/onlinejudge/showProblem.do?problemId=2
Supposethatwehaveasquarecitywithstraightstreets.Amapofacityisasquareboardwithnrowsandncolumns,eachrepresentingastreetorapieceofwall.Ablockhouseisasmallcastlethathasfouropeningsthroughwhichtoshoot.Thef