View Compilation Error

0_0_28523067_13516.cpp: In function 'int main()':
0_0_28523067_13516.cpp:6:21: error: expression cannot be used as a function
          l=(a+c)(a-c)+(b+d)(b-d);
                     ^
0_0_28523067_13516.cpp:6:32: error: expression cannot be used as a function
          l=(a+c)(a-c)+(b+d)(b-d);
                                ^