foo.c: In function ‘main’:
foo.c:13:30: error: ‘i’ undeclared (first use in this function)
   13 |                 ret=ret*10+a[i];
      |                              ^
foo.c:13:30: note: each undeclared identifier is reported only once for each function it appears in
foo.c:15:9: error: ‘number’ undeclared (first use in this function)
   15 |         number++;
      |         ^~~~~~