View Compilation Error

0_0_23772736_14624.cpp: In function 'int main()':
0_0_23772736_14624.cpp:20:8: error: 'M_PI' was not declared in this scope
    out(M_PI * Sqr(r1));
        ^
0_0_23772736_14624.cpp:10:29: note: in definition of macro 'out'
 #define out(x) std::cout << x << std::endl
                             ^
0_0_23772736_14624.cpp:26:60: error: 'M_PI' was not declared in this scope
     out(Alp * r1 * r1 + Bet * r2 * r2 - r1 * r2 * std::sin(M_PI - Alp - Bet));
                                                            ^
0_0_23772736_14624.cpp:10:29: note: in definition of macro 'out'
 #define out(x) std::cout << x << std::endl
                             ^