View Compilation Error

0_0_16379655_28301.c: In function 'main':
0_0_16379655_28301.c:5:42: error: called object 'a' is not a function or function pointer
   while(scanf("%d",&a)!=EOF){printf("%d",a(1+a)/2);}
                                          ^
0_0_16379655_28301.c:4:7: note: declared here
   int a;
       ^