View Compilation Error

0_0_29107956_3197.cpp: In function 'int main()':
0_0_29107956_3197.cpp:16:13: error: 'PI' was not declared in this scope
         C=2*PI*R;
             ^
0_0_29107956_3197.cpp:23:31: error: unable to find numeric literal operator 'operator""acos'
                     double r1=1acos((l*l+r*r-R*R)/(2.0*l*r));
                               ^
0_0_29107956_3197.cpp:23:31: note: use -std=gnu++11 or -fext-numeric-literals to enable more built-in suffixes
0_0_29107956_3197.cpp:29:27: error: 'pai' was not declared in this scope
                     C=C+2*pai*r;
                           ^