View Compilation Error

0_0_29735171_10996.cpp: In function 'int main()':
0_0_29735171_10996.cpp:13:40: error: 'p' was not declared in this scope
   ans-=(q*q+r*r-l*l)/2*(q*q+r*r-l*l)/2*p*p;    
                                        ^