Language: C++

#include <iostream>
#include <fstream>
#include <stdio.h>
#include <vector>
#include <string>
#include <math.h>
#include <algorithm>
#include <map>
#include <set>

#define sz(x) ((int)x.size())
#define all(x) (x).begin(), (x).end()
#define pb(x) push_back(x)
#define mp(x, y) make_pair(x, y)

typedef long long int64;

typedef lld lld;

using namespace std;


// https://atcoder.jp/contests/abc151/tasks/abc151_f

lld _abs(lld x) {
    if (x < 0) return -x;
    return x;
}

lld sqr(lld x) {
   return x * x;
}

lld det(const vector<vector<lld>> a) {
    return a[0][0] * (a[1][1] * a[2][2] - a[1][2] * a[2][1]) -
           a[1][0] * (a[0][1] * a[2][2] - a[2][1] * a[0][2]) +
           a[2][0] * (a[0][1] * a[1][2] - a[0][2] * a[1][1]);
}

pair<lld, lld> find_center(lld x1, lld y1, lld x2, lld y2, lld x3, lld y3) {
    // https://en.wikipedia.org/wiki/Circumscribed_circle#Cartesian_coordinates
    lld len1 = sqr(x1) + sqr(y1);
    lld len2 = sqr(x2) + sqr(y2);
    lld len3 = sqr(x3) + sqr(y3);    
    vector<vector<lld>> a = { {len1, y1, 1}, {len2, y2, 1}, {len3, y3, 1} };
    vector<vector<lld>> b = { {x1, len1, 1}, {x2, len2, 1}, {x3, len3, 1} };
    vector<vector<lld>> c = { {x1, y1, 1}, {x2, y2, 1}, {x3, y3, 1} };
    lld sx = 0.5 * det(a);
    lld sy = 0.5 * det(b);
    lld A = det(c);
    if (_abs(A) < 1e-9) return {0, 0};
    sx /= A;
    sy /= A;
    return {sx, sy};
}    

int main() {
    ios_base::sync_with_stdio(0);

    int n;
    cin >> n;
    vector<lld> x(n);
    vector<lld> y(n);
    
    for (int i = 0; i < n; ++i) {
        cin >> x[i] >> y[i];
    }
    
    lld ans = -1;
    for (int i = 0; i < n; ++i) {
        for (int j = i + 1; j < n;  ++j) {
            lld xr = (x[i] + x[j]) / 2;
            lld yr = (y[i] + y[j]) / 2;
            lld res = 0;
            for (int l = 0; l < n; ++l) {
                res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
            }
            if (ans < 0 || ans >= res) {
                ans = res;
            }
            
            for (int k = j + 1; k < n; ++k) {
                auto p = find_center(x[i], y[i], x[j], y[j], x[k], y[k]);
                lld xr = p.first;
                lld yr = p.second;
                lld res = 0;
            
                for (int l = 0; l < n; ++l) {
                      res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
                }
                if (ans < 0 || ans >= res) {
                    ans = res;
                }
            }
        }
    }

    cout.precision(15);
    cout << sqrt(ans) << endl;

    return 0;
}

Compiler output:
a.cc:18:9: error: 'lld' does not name a type
   18 | typedef lld lld;
      |         ^~~
a.cc:25:1: error: 'lld' does not name a type
   25 | lld _abs(lld x) {
      | ^~~
a.cc:30:1: error: 'lld' does not name a type
   30 | lld sqr(lld x) {
      | ^~~
a.cc:34:1: error: 'lld' does not name a type
   34 | lld det(const vector<vector<lld>> a) {
      | ^~~
a.cc:40:6: error: 'lld' was not declared in this scope
   40 | pair<lld, lld> find_center(lld x1, lld y1, lld x2, lld y2, lld x3, lld y3) {
      |      ^~~
a.cc:40:11: error: 'lld' was not declared in this scope
   40 | pair<lld, lld> find_center(lld x1, lld y1, lld x2, lld y2, lld x3, lld y3) {
      |           ^~~
a.cc:40:14: error: template argument 1 is invalid
   40 | pair<lld, lld> find_center(lld x1, lld y1, lld x2, lld y2, lld x3, lld y3) {
      |              ^
a.cc:40:14: error: template argument 2 is invalid
a.cc:40:28: error: 'lld' was not declared in this scope
   40 | pair<lld, lld> find_center(lld x1, lld y1, lld x2, lld y2, lld x3, lld y3) {
      |                            ^~~
a.cc:40:36: error: 'lld' was not declared in this scope
   40 | pair<lld, lld> find_center(lld x1, lld y1, lld x2, lld y2, lld x3, lld y3) {
      |                                    ^~~
a.cc:40:44: error: 'lld' was not declared in this scope
   40 | pair<lld, lld> find_center(lld x1, lld y1, lld x2, lld y2, lld x3, lld y3) {
      |                                            ^~~
a.cc:40:52: error: 'lld' was not declared in this scope
   40 | pair<lld, lld> find_center(lld x1, lld y1, lld x2, lld y2, lld x3, lld y3) {
      |                                                    ^~~
a.cc:40:60: error: 'lld' was not declared in this scope
   40 | pair<lld, lld> find_center(lld x1, lld y1, lld x2, lld y2, lld x3, lld y3) {
      |                                                            ^~~
a.cc:40:68: error: 'lld' was not declared in this scope
   40 | pair<lld, lld> find_center(lld x1, lld y1, lld x2, lld y2, lld x3, lld y3) {
      |                                                                    ^~~
a.cc:40:74: error: expression list treated as compound expression in initializer [-fpermissive]
   40 | pair<lld, lld> find_center(lld x1, lld y1, lld x2, lld y2, lld x3, lld y3) {
      |                                                                          ^
a.cc: In function 'int main()':
a.cc:62:12: error: 'lld' was not declared in this scope
   62 |     vector<lld> x(n);
      |            ^~~
a.cc:62:15: error: template argument 1 is invalid
   62 |     vector<lld> x(n);
      |               ^
a.cc:62:15: error: template argument 2 is invalid
a.cc:63:15: error: template argument 2 is invalid
   63 |     vector<lld> y(n);
      |               ^
a.cc:66:17: error: invalid types 'int[int]' for array subscript
   66 |         cin >> x[i] >> y[i];
      |                 ^
a.cc:66:25: error: invalid types 'int[int]' for array subscript
   66 |         cin >> x[i] >> y[i];
      |                         ^
a.cc:69:8: error: expected ';' before 'ans'
   69 |     lld ans = -1;
      |        ^~~~
      |        ;
a.cc:72:16: error: expected ';' before 'xr'
   72 |             lld xr = (x[i] + x[j]) / 2;
      |                ^~~
      |                ;
a.cc:73:16: error: expected ';' before 'yr'
   73 |             lld yr = (y[i] + y[j]) / 2;
      |                ^~~
      |                ;
a.cc:74:16: error: expected ';' before 'res'
   74 |             lld res = 0;
      |                ^~~~
      |                ;
a.cc:76:17: error: 'res' was not declared in this scope
   76 |                 res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
      |                 ^~~
a.cc:76:36: error: 'xr' was not declared in this scope; did you mean 'x'?
   76 |                 res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
      |                                    ^~
      |                                    x
a.cc:76:42: error: invalid types 'int[int]' for array subscript
   76 |                 res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
      |                                          ^
a.cc:76:32: error: 'sqr' was not declared in this scope; did you mean 'sqrt'?
   76 |                 res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
      |                                ^~~
      |                                sqrt
a.cc:76:53: error: 'yr' was not declared in this scope; did you mean 'y'?
   76 |                 res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
      |                                                     ^~
      |                                                     y
a.cc:76:59: error: invalid types 'int[int]' for array subscript
   76 |                 res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
      |                                                           ^
a.cc:78:17: error: 'ans' was not declared in this scope; did you mean 'abs'?
   78 |             if (ans < 0 || ans >= res) {
      |                 ^~~
      |                 abs
a.cc:78:35: error: 'res' was not declared in this scope
   78 |             if (ans < 0 || ans >= res) {
      |                                   ^~~
a.cc:83:39: error: invalid types 'int[int]' for array subscript
   83 |                 auto p = find_center(x[i], y[i], x[j], y[j], x[k], y[k]);
      |                                       ^
a.cc:83:45: error: invalid types 'int[int]' for array subscript
   83 |                 auto p = find_center(x[i], y[i], x[j], y[j], x[k], y[k]);
      |                                             ^
a.cc:83:51: error: invalid types 'int[int]' for array subscript
   83 |                 auto p = find_center(x[i], y[i], x[j], y[j], x[k], y[k]);
      |                                                   ^
a.cc:83:57: error: invalid types 'int[int]' for array subscript
   83 |                 auto p = find_center(x[i], y[i], x[j], y[j], x[k], y[k]);
      |                                                         ^
a.cc:83:63: error: invalid types 'int[int]' for array subscript
   83 |                 auto p = find_center(x[i], y[i], x[j], y[j], x[k], y[k]);
      |                                                               ^
a.cc:83:69: error: invalid types 'int[int]' for array subscript
   83 |                 auto p = find_center(x[i], y[i], x[j], y[j], x[k], y[k]);
      |                                                                     ^
a.cc:83:72: error: 'find_center' cannot be used as a function
   83 |                 auto p = find_center(x[i], y[i], x[j], y[j], x[k], y[k]);
      |                                                                        ^
a.cc:84:20: error: expected ';' before 'xr'
   84 |                 lld xr = p.first;
      |                    ^~~
      |                    ;
a.cc:85:20: error: expected ';' before 'yr'
   85 |                 lld yr = p.second;
      |                    ^~~
      |                    ;
a.cc:86:20: error: expected ';' before 'res'
   86 |                 lld res = 0;
      |                    ^~~~
      |                    ;
a.cc:89:23: error: 'res' was not declared in this scope
   89 |                       res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
      |                       ^~~
a.cc:89:42: error: 'xr' was not declared in this scope; did you mean 'x'?
   89 |                       res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
      |                                          ^~
      |                                          x
a.cc:89:48: error: invalid types 'int[int]' for array subscript
   89 |                       res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
      |                                                ^
a.cc:89:38: error: 'sqr' was not declared in this scope; did you mean 'sqrt'?
   89 |                       res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
      |                                      ^~~
      |                                      sqrt
a.cc:89:59: error: 'yr' was not declared in this scope; did you mean 'y'?
   89 |                       res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
      |                                                           ^~
      |                                                           y
a.cc:89:65: error: invalid types 'int[int]' for array subscript
   89 |                       res = max(res, sqr(xr - x[l]) + sqr(yr - y[l]));
      |                                                                 ^
a.cc:91:21: error: 'ans' was not declared in this scope; did you mean 'abs'?
   91 |                 if (ans < 0 || ans >= res) {
      |                     ^~~
      |                     abs
a.cc:91:39: error: 'res' was not declared in this scope
   91 |                 if (ans < 0 || ans >= res) {
      |                                       ^~~
a.cc:99:18: error: 'ans' was not declared in this scope; did you mean 'abs'?
   99 |     cout << sqrt(ans) << endl;
      |                  ^~~
      |                  abs