/in/foo.c:3:6: warning: return type of 'main' is not 'int' [-Wmain]
 void main() {
      ^~~~
/in/foo.c: In function 'main':
/in/foo.c:4:9: warning: unused variable 'a' [-Wunused-variable]
     int a,i,k,sum,n,t=0;
         ^