Language: C++

#include <bits/stdc++.h>
using namespace std;
//http://blog.livedoor.jp/dgd17/archives/11821338.html
inline complex<double> Circumcenter(complex<double> a, complex<double> b, complex<double> c) {

	//辺
	double eAB = abs(b - a);
	double eBC = abs(c - b);
	double eCA = abs(a - c);

	//角度(余弦定理)
	double aCAB = acos((eCA*eCA + eAB*eAB - eBC*eBC) / (2 * eCA*eAB));
	double aABC = acos((eAB*eAB + eBC*eBC - eCA*eCA) / (2 * eAB*eBC));
	double aBCA = acos((eBC*eBC + eCA*eCA - eAB*eAB) / (2 * eBC*eCA));

	double A = sin(2 * aCAB);
	double B = sin(2 * aABC);
	double C = sin(2 * aBCA);

	//外心の位置ベクトル(ベクトルの面積比の公式)
	complex<double> ans;
	ans.real((A*a.real() + B*b.real() + C*c.real()) / (A + B + C));
	ans.imag((A*a.imag() + B*b.imag() + C*c.imag()) / (A + B + C));
	return(ans);
} 

inline double Circumradius(complex<double> a, complex<double> b, complex<double> c) {

	//辺
	double eAB = abs(b - a);
	double eBC = abs(c - b);
	double eCA = abs(a - c);

	//角度(余弦定理)
	double aCAB = acos((eCA*eCA + eAB*eAB - eBC*eBC) / (2 * eCA*eAB));

	//外接円の半径(正弦定理)
	double R = 0.5*eBC / sin(aCAB);

	return(R);

}

int main(void){
  cout << fixed << setprecision(20);
  int N;
  cin >> N;
  vector<int> x(N), y(N);
  for (int i = 0; i < N; i++){
    cin >> x[i] >> y[i];
  }
  vector<complex<double>> c(N);
  for (int i = 0; i < N; i++){
    c[i].real(x[i]);
    c[i].imag(y[i]);
  }
  if (N == 2){
    cout << abs(c[0]-c[1])/2;
  } else {
    double minrad = 10000;
    complex<double> cc;
    double cr;
    //2点
    for (int i = 0; i < N; i++){
      for (int j = 0; j < N; j++){
        cc = (c[i]+c[j])/2;
        cr = abs((c[j]-c[i])/2);
        if (cr < minrad){
          bool flg = true;
          for (int n = 0; n < N; n++){
            if (abs(cc - c[n]) > cr + 1e-7) flg = false;
          }
          if (flg) minrad = cr;
        }
      }
    }   
    //3点
    for (int i = 0; i < N; i++){
      for (int j = i + 1; j < N; j++){
        for (int k = j + 1; k < N; k++){
          cc = Circumcenter(c[i],c[j],c[k]);
          cr = Circumradius(c[i],c[j],c[k]);
          if (cr < minrad){
            bool flg = true;
            for (int n = 0; n < N; n++){
              if (abs(cc - c[n]) > cr + 1e-7) flg = false;
            }
            if (flg) minrad = cr;
          }
        }
      }
    }
    cout << minrad;
  }
}

Compiler output:
a.cc: In function 'int main()':
a.cc:66:25: error: no match for 'operator/' (operand types are 'std::complex<double>' and 'int')
   66 |         cc = (c[i]+c[j])/2;
      |              ~~~~~~~~~~~^~
      |                   |      |
      |                   |      int
      |                   std::complex<double>
In file included from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127,
                 from a.cc:1:
/usr/include/c++/14/complex:430:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator/(const complex<_Tp>&, const complex<_Tp>&)'
  430 |     operator/(const complex<_Tp>& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:430:5: note:   template argument deduction/substitution failed:
a.cc:66:26: note:   mismatched types 'const std::complex<_Tp>' and 'int'
   66 |         cc = (c[i]+c[j])/2;
      |                          ^
/usr/include/c++/14/complex:439:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator/(const complex<_Tp>&, const _Tp&)'
  439 |     operator/(const complex<_Tp>& __x, const _Tp& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:439:5: note:   template argument deduction/substitution failed:
a.cc:66:26: note:   deduced conflicting types for parameter 'const _Tp' ('double' and 'int')
   66 |         cc = (c[i]+c[j])/2;
      |                          ^
/usr/include/c++/14/complex:448:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator/(const _Tp&, const complex<_Tp>&)'
  448 |     operator/(const _Tp& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:448:5: note:   template argument deduction/substitution failed:
a.cc:66:26: note:   mismatched types 'const std::complex<_Tp>' and 'int'
   66 |         cc = (c[i]+c[j])/2;
      |                          ^
In file included from /usr/include/c++/14/valarray:605,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:166:
/usr/include/c++/14/bits/valarray_after.h:408:5: note: candidate: 'template<class _Dom1, class _Dom2> std::_Expr<std::__detail::_BinClos<std::__divides, std::_Expr, std::_Expr, _Dom1, _Dom2>, typename std::__fun<std::__divides, typename _Dom1::value_type>::result_type> std::operator/(const _Expr<_Dom1, typename _Dom1::value_type>&, const _Expr<_Dom2, typename _Dom2::value_type>&)'
  408 |     _DEFINE_EXPR_BINARY_OPERATOR(/, struct std::__divides)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:408:5: note:   template argument deduction/substitution failed:
a.cc:66:26: note:   'std::complex<double>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   66 |         cc = (c[i]+c[j])/2;
      |                          ^
/usr/include/c++/14/bits/valarray_after.h:408:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__divides, std::_Expr, std::_Constant, _Dom, typename _Dom::value_type>, typename std::__fun<std::__divides, typename _Dom1::value_type>::result_type> std::operator/(const _Expr<_Dom1, typename _Dom1::value_type>&, const typename _Dom::value_type&)'
  408 |     _DEFINE_EXPR_BINARY_OPERATOR(/, struct std::__divides)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:408:5: note:   template argument deduction/substitution failed:
a.cc:66:26: note:   'std::complex<double>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   66 |         cc = (c[i]+c[j])/2;
      |                          ^
/usr/include/c++/14/bits/valarray_after.h:408:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__divides, std::_Constant, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__divides, typename _Dom1::value_type>::result_type> std::operator/(const typename _Dom::value_type&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  408 |     _DEFINE_EXPR_BINARY_OPERATOR(/, struct std::__divides)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:408:5: note:   template argument deduction/substitution failed:
a.cc:66:26: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'int'
   66 |         cc = (c[i]+c[j])/2;
      |                          ^
/usr/include/c++/14/bits/valarray_after.h:408:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__divides, std::_Expr, std::_ValArray, _Dom, typename _Dom::value_type>, typename std::__fun<std::__divides, typename _Dom1::value_type>::result_type> std::operator/(const _Expr<_Dom1, typename _Dom1::value_type>&, const valarray<typename _Dom::value_type>&)'
  408 |     _DEFINE_EXPR_BINARY_OPERATOR(/, struct std::__divides)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:408:5: note:   template argument deduction/substitution failed:
a.cc:66:26: note:   'std::complex<double>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   66 |         cc = (c[i]+c[j])/2;
      |                          ^
/usr/include/c++/14/bits/valarray_after.h:408:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__divides, std::_ValArray, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__divides, typename _Dom1::value_type>::result_type> std::operator/(const valarray<typename _Dom::value_type>&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  408 |     _DEFINE_EXPR_BINARY_OPERATOR(/, struct std::__divides)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:408:5: note:   template argument deduction/substitution failed:
a.cc:66:26: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'int'
   66 |         cc = (c[i]+c[j])/2;
      |                          ^
/usr/include/c++/14/valarray:1199:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__divides, std::_ValArray, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__divides, _Tp>::result_type> std::operator/(const valarray<_Tp>&, const valarray<_Tp>&)'
 1199 | _DEFINE_BINARY_OPERATOR(/, __divides)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1199:1: note:   template argument deduction/substitution failed:
a.cc:66:26: note:   'std::complex<double>' is not derived from 'const std::valarray<_Tp>'
   66 |         cc = (c[i]+c[j])/2;
      |                          ^
/usr/include/c++/14/valarray:1199:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__divides, std::_ValArray, std::_Constant, _Tp, _Tp>, typename std::__fun<std::__divides, _Tp>::result_type> std::operator/(const valarray<_Tp>&, const typename valarray<_Tp>::value_type&)'
 1199 | _DEFINE_BINARY_OPERATOR(/, __divides)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1199:1: note:   template argument deduction/substitution failed:
a.cc:66:26: note:   'std::complex<double>' is not derived from 'const std::valarray<_Tp>'
   66 |         cc = (c[i]+c[j])/2;
      |                          ^
/usr/include/c++/14/valarray:1199:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__divides, std::_Constant, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__divides, _Tp>::result_type> std::operator/(const typename valarray<_Tp>::value_type&, const valarray<_Tp>&)'
 1199 | _DEFINE_BINARY_OPERATOR(/, __divides)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1199:1: note:   template argument deduction/substitution failed:
a.cc:66:26: note:   mismatched types 'const std::valarray<_Tp>' and 'int'
   66 |         cc = (c[i]+c[j])/2;
      |                          ^
a.cc:67:29: error: no match for 'operator/' (operand types are 'std::complex<double>' and 'int')
   67 |         cr = abs((c[j]-c[i])/2);
      |                  ~~~~~~~~~~~^~
      |                       |      |
      |                       |      int
      |                       std::complex<double>
/usr/include/c++/14/complex:430:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator/(const complex<_Tp>&, const complex<_Tp>&)'
  430 |     operator/(const complex<_Tp>& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:430:5: note:   template argument deduction/substitution failed:
a.cc:67:30: note:   mismatched types 'const std::complex<_Tp>' and 'int'
   67 |         cr = abs((c[j]-c[i])/2);
      |                              ^
/usr/include/c++/14/complex:439:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator/(const complex<_Tp>&, const _Tp&)'
  439 |     operator/(const complex<_Tp>& __x, const _Tp& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:439:5: note:   template argument deduction/substitution failed:
a.cc:67:30: note:   deduced conflicting types for parameter 'const _Tp' ('double' and 'int')
   67 |         cr = abs((c[j]-c[i])/2);
      |                              ^
/usr/include/c++/14/complex:448:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator/(const _Tp&, const complex<_Tp>&)'
  448 |     operator/(const _Tp& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:448:5: note:   template argument deduction/substitution failed:
a.cc:67:30: note:   mismatched types 'const std::complex<_Tp>' and 'int'
   67 |         cr = abs((c[j]-c[i])/2);
      |                              ^
/usr/include/c++/14/bits/valarray_after.h:408:5: note: candidate: 'template<class _Dom1, class _Dom2> std::_Expr<std::__detail::_BinClos<std::__divides, std::_Expr, std::_Expr, _Dom1, _Dom2>, typename std::__fun<std::__divides, typename _Dom1::value_type>::result_type> std::operator/(const _Expr<_Dom1, typename _Dom1::value_type>&, const _Expr<_Dom2, typename _Dom2::value_type>&)'
  408 |     _DEFINE_EXPR_BINARY_OPERATOR(/, struct std::__divides)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:408:5: note:   template argument deduction/substitution failed:
a.cc:67:30: note:   'std::complex<double>' is not derived from 'const std::_Expr<_Dom