la-bl+lc-al=1la-bl=1,lc-al=0或la-bl=0,lc-al=1当la-bl=1,lc-al=0时a=cla-bl+lb-cl+lc-al=1+lb-cl=1+lb-al=2当la-bl=0,lc-al=1时a=bla-bl+lb-cl+lc-al=1+lb-cl=1+la-cl=2
