View Compilation Error

0_0_35647085_14932.c: In function 'sum':
0_0_35647085_14932.c:4:9: error: called object 'a' is not a function or function pointer
  return a(a+1)/2;
         ^
0_0_35647085_14932.c:3:13: note: declared here
 int sum(int a){
             ^