/in/foo.c: In function 'main':
/in/foo.c:6:13: error: 'i' undeclared (first use in this function)
    6 |         for(i=1;i<=1000;i++){
      |             ^
/in/foo.c:6:13: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:7:17: error: 'sum' undeclared (first use in this function)
    7 |                 sum=0;
      |                 ^~~
/in/foo.c:8:21: error: 'j' undeclared (first use in this function)
    8 |                 for(j=2;j<n;j++){
      |                     ^