/in/foo.c: In function 'main':
/in/foo.c:7:1: error: unknown type name '_int64'
 _int64 sum=0;
 ^~~~~~
/in/foo.c:9:2: error: unknown type name '_int64'
  _int64 s=1;
  ^~~~~~
/in/foo.c:17:7: error: unknown type name '_int64'
   for(_int64 i=1;i<=s/2;i++){
       ^~~~~~