Language: C++

#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <limits>#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <vector>

#define len(X) ((int)(X).size())

#ifdef __LOCAL
#define DBG(X) cout << #X << "=" << (X) << '\n';
#define SAY(X) cout << (X) << '\n';
#else
#define DBG(X)
#define SAY(X)
#endif

#define M_PI       3.14159265358979323846

using namespace std;

using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

const int INT_INF = (int)(2e9);
const ll  LL_INF = (ll)(2e18);

const int NIL = -1;
static mt19937 _g(time(nullptr));

std::string pad(int num)
{
    char buffer[4];
    std::snprintf(buffer, sizeof(buffer), "%03d", num);
    return buffer;
}

inline ll randint(ll a, ll b) { ll w = (_g() << 31LL) ^ _g(); return a + w % (b - a + 1); }
inline void fast_io() { ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr); };
template<typename T> inline T sign(T x) { return T(x > 0) - T(x < 0); }
template<typename T, typename S> inline ostream& operator<<(ostream& os, const pair<T, S> p) { cout << "[" << p.first << ";" << p.second << "]"; return os; }
template<typename T> inline ostream& operator<<(ostream& os, const vector<T>& v) { for (auto el : v) cout << el << " "; return os; }
template<typename T> inline T fetch() { T ret; cin >> ret; return ret; }
template<typename T> inline vector<T> fetch_vec(int sz) { vector<T> ret(sz); for (auto& elem : ret) cin >> elem; return ret; }


int solve()
{
    int L, R;
    const int MOD = 2019;

    cin >> L >> R;

    if (R - L  + 1 >= MOD)
    {
        return 0;
    }
    else
    {
        int min = 3000;
        int secondMin = 3001;

        for (int i = L; i <= R; ++i)
        {
            if (i % MOD < min)
            {
                secondMin = min;
                min = i % MOD;
            }
            else if (i % MOD < secondMin)
            {
                secondMin = i % MOD;
            }
        }

        return min * secondMin;
    }
}

int main()
{
    std::cout << solve() << std::endl;

    return 0;
}
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <vector>

#define len(X) ((int)(X).size())

#ifdef __LOCAL
#define DBG(X) cout << #X << "=" << (X) << '\n';
#define SAY(X) cout << (X) << '\n';
#else
#define DBG(X)
#define SAY(X)
#endif

#define M_PI       3.14159265358979323846

using namespace std;

using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

const int INT_INF = (int)(2e9);
const ll  LL_INF = (ll)(2e18);

const int NIL = -1;
static mt19937 _g(time(nullptr));

std::string pad(int num)
{
    char buffer[4];
    std::snprintf(buffer, sizeof(buffer), "%03d", num);
    return buffer;
}

inline ll randint(ll a, ll b) { ll w = (_g() << 31LL) ^ _g(); return a + w % (b - a + 1); }
inline void fast_io() { ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr); };
template<typename T> inline T sign(T x) { return T(x > 0) - T(x < 0); }
template<typename T, typename S> inline ostream& operator<<(ostream& os, const pair<T, S> p) { cout << "[" << p.first << ";" << p.second << "]"; return os; }
template<typename T> inline ostream& operator<<(ostream& os, const vector<T>& v) { for (auto el : v) cout << el << " "; return os; }
template<typename T> inline T fetch() { T ret; cin >> ret; return ret; }
template<typename T> inline vector<T> fetch_vec(int sz) { vector<T> ret(sz); for (auto& elem : ret) cin >> elem; return ret; }


int solve()
{
    int L, R;
    const int MOD = 2019;

    cin >> L >> R;

    if (R - L >= MOD - 1)
    {
        return 0;
    }
    else
    {
        int min = 3000;
        int secondMin = 3001;

        for (int i = L; i <= R; ++i)
        {
            if (i % MOD < min)
            {
                secondMin = min;
                min = i % MOD;
            }
            else if (i % MOD < secondMin)
            {
                secondMin = i % MOD;
            }
        }

        return min * secondMin;
    }
}

int main()
{
    std::cout << solve() << std::endl;

    return 0;
}

Compiler output:
a.cc:11:18: warning: extra tokens at end of #include directive
   11 | #include <limits>#include <algorithm>
      |                  ^
a.cc:145:11: error: redefinition of 'const int INT_INF'
  145 | const int INT_INF = (int)(2e9);
      |           ^~~~~~~
a.cc:53:11: note: 'const int INT_INF' previously defined here
   53 | const int INT_INF = (int)(2e9);
      |           ^~~~~~~
a.cc:146:11: error: redefinition of 'const ll LL_INF'
  146 | const ll  LL_INF = (ll)(2e18);
      |           ^~~~~~
a.cc:54:11: note: 'const ll LL_INF' previously defined here
   54 | const ll  LL_INF = (ll)(2e18);
      |           ^~~~~~
a.cc:148:11: error: redefinition of 'const int NIL'
  148 | const int NIL = -1;
      |           ^~~
a.cc:56:11: note: 'const int NIL' previously defined here
   56 | const int NIL = -1;
      |           ^~~
a.cc:149:16: error: redefinition of 'std::mt19937 _g'
  149 | static mt19937 _g(time(nullptr));
      |                ^~
a.cc:57:16: note: 'std::mt19937 _g' previously declared here
   57 | static mt19937 _g(time(nullptr));
      |                ^~
a.cc:151:13: error: redefinition of 'std::string pad(int)'
  151 | std::string pad(int num)
      |             ^~~
a.cc:59:13: note: 'std::string pad(int)' previously defined here
   59 | std::string pad(int num)
      |             ^~~
a.cc:158:11: error: redefinition of 'll randint(ll, ll)'
  158 | inline ll randint(ll a, ll b) { ll w = (_g() << 31LL) ^ _g(); return a + w % (b - a + 1); }
      |           ^~~~~~~
a.cc:66:11: note: 'll randint(ll, ll)' previously defined here
   66 | inline ll randint(ll a, ll b) { ll w = (_g() << 31LL) ^ _g(); return a + w % (b - a + 1); }
      |           ^~~~~~~
a.cc:159:13: error: redefinition of 'void fast_io()'
  159 | inline void fast_io() { ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr); };
      |             ^~~~~~~
a.cc:67:13: note: 'void fast_io()' previously defined here
   67 | inline void fast_io() { ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr); };
      |             ^~~~~~~
a.cc:160:31: error: redefinition of 'template<class T> T sign(T)'
  160 | template<typename T> inline T sign(T x) { return T(x > 0) - T(x < 0); }
      |                               ^~~~
a.cc:68:31: note: 'template<class T> T sign(T)' previously declared here
   68 | template<typename T> inline T sign(T x) { return T(x > 0) - T(x < 0); }
      |                               ^~~~
a.cc:161:50: error: redefinition of 'template<class T, class S> std::ostream& operator<<(std::ostream&, std::pair<_T1, _T2>)'
  161 | template<typename T, typename S> inline ostream& operator<<(ostream& os, const pair<T, S> p) { cout << "[" << p.first << ";" << p.second << "]"; return os; }
      |                                                  ^~~~~~~~
a.cc:69:50: note: 'template<class T, class S> std::ostream& operator<<(std::ostream&, std::pair<_T1, _T2>)' previously declared here
   69 | template<typename T, typename S> inline ostream& operator<<(ostream& os, const pair<T, S> p) { cout << "[" << p.first << ";" << p.second << "]"; return os; }
      |                                                  ^~~~~~~~
a.cc:162:38: error: redefinition of 'template<class T> std::ostream& operator<<(std::ostream&, const std::vector<_Tp>&)'
  162 | template<typename T> inline ostream& operator<<(ostream& os, const vector<T>& v) { for (auto el : v) cout << el << " "; return os; }
      |                                      ^~~~~~~~
a.cc:70:38: note: 'template<class T> std::ostream& operator<<(std::ostream&, const std::vector<_Tp>&)' previously declared here
   70 | template<typename T> inline ostream& operator<<(ostream& os, const vector<T>& v) { for (auto el : v) cout << el << " "; return os; }
      |                                      ^~~~~~~~
a.cc:163:31: error: redefinition of 'template<class T> T fetch()'
  163 | template<typename T> inline T fetch() { T ret; cin >> ret; return ret; }
      |                               ^~~~~
a.cc:71:31: note: 'template<class T> T fetch()' previously declared here
   71 | template<typename T> inline T fetch() { T ret; cin >> ret; return ret; }
      |                               ^~~~~
a.cc:164:39: error: redefinition of 'template<class T> std::vector<_Tp> fetch_vec(int)'
  164 | template<typename T> inline vector<T> fetch_vec(int sz) { vector<T> ret(sz); for (auto& elem : ret) cin >> elem; return ret; }
      |                                       ^~~~~~~~~
a.cc:72:39: note: 'template<class T> std::vector<_Tp> fetch_vec(int)' previously declared here
   72 | template<typename T> inline vector<T> fetch_vec(int sz) { vector<T> ret(sz); for (auto& elem : ret) cin >> elem; return ret; }
      |                                       ^~~~~~~~~
a.cc:167:5: error: redefinition of 'int solve()'
  167 | int solve()
      |     ^~~~~
a.cc:75:5: note: 'int solve()' previously defined here
   75 | int solve()
      |     ^~~~~
a.cc:200:5: error: redefinition of 'int main()'
  200 | int main()
      |     ^~~~
a.cc:108:5: note: 'int main()' previously defined here
  108 | int main()
      |     ^~~~