/in/foo.c: In function 'main':
/in/foo.c:6:8: warning: too many arguments for format [-Wformat-extra-args]
  scanf("%f",&a,&b);
        ^~~~
/in/foo.c:7:6: warning: 'PI' is used uninitialized in this function [-Wuninitialized]
  S=PI*a*a;
    ~~^~