View Compilation Error

0_0_19131362_6462.cpp: In function 'int main()':
0_0_19131362_6462.cpp:28:23: error: 'fbs' was not declared in this scope
         xx=fbs(a.x-b.x)+fbs(c.x-d.x)-mmax(fbs(a.x-c.x),fbs(a.x-d.x),fbs(b.x-c.x),fbs(b.x-d.x));
                       ^