/in/foo.c: In function 'main':
/in/foo.c:5:15: error: expected identifier or '(' before 'long'
  int a[100],n,long s,t,i;
               ^~~~
/in/foo.c:7:6: error: 'i' undeclared (first use in this function)
  for(i=0;i<n;i++)
      ^
/in/foo.c:7:6: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:9:2: error: 's' undeclared (first use in this function)
  s=a[0];
  ^
/in/foo.c:12:3: error: 't' undeclared (first use in this function)
   t=a[i+1];
   ^