/in/foo.c: In function 'main':
/in/foo.c:8:17: error: 'o' undeclared (first use in this function)
    8 |         int sum=o;
      |                 ^
/in/foo.c:8:17: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:12:17: error: 'pingfang' undeclared (first use in this function); did you mean 'pinfang'?
   12 |         chengji=pingfang*i*i;
      |                 ^~~~~~~~
      |                 pinfang
/in/foo.c:7:13: warning: unused variable 'pinfang' [-Wunused-variable]
    7 |         int pinfang=1;
      |             ^~~~~~~