View Compilation Error

0_0_17888241_30776.cpp:7:25: error: expected unqualified-id before numeric constant
 const double Pi2=Pi*0.5,2Pi=Pi*2.0;
                         ^
0_0_17888241_30776.cpp: In function 'double dif(const double&, const double&)':
0_0_17888241_30776.cpp:20:8: error: unable to find numeric literal operator 'operator""Pi'
   ans+=2Pi;
        ^
0_0_17888241_30776.cpp:20:8: note: use -std=gnu++11 or -fext-numeric-literals to enable more built-in suffixes