/in/foo.c: In function 'main':
/in/foo.c:11:5: error: expected ';' before 'for'
     for(i=4;i<=n;i++)
     ^~~
/in/foo.c:6:13: warning: variable 'f' set but not used [-Wunused-but-set-variable]
     int n,i,f[1000002]={0};
             ^
/in/foo.c:6:11: warning: unused variable 'i' [-Wunused-variable]
     int n,i,f[1000002]={0};
           ^