View Compilation Error

0_0_24103204_11631.c: In function 'main':
0_0_24103204_11631.c:11:29: error: 'M_PI' undeclared (first use in this function)
         s+=(int)((0.5*log(2*M_PI*n)+n*log(n)-n)/log(10.0));
                             ^
0_0_24103204_11631.c:11:29: note: each undeclared identifier is reported only once for each function it appears in