Problem p03836 (C++)

Buggy submission (status: Runtime Error):
#define _GLIBCXX_DEBUG
#include <algorithm> //next_permutation
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); i++)
#define all(v) v.begin(), v.end()
#define dec(n) cout << fixed << setprecision(n);
#define large "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
#define small "abcdefghijklmnopqrstuvwxyz"
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
using vl = vector<ll>;
using vvl = vector<vl>;

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

const ll MOD = 1000000007;
const ll MAX = 2000001;

ll mod(ll a) { return a % MOD; }

ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }

ll fac[MAX], finv[MAX], inv[MAX];

void nCrprep() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (ll i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

ll nCr(ll n, ll r) {
  if (n < r)
    return 0;
  if (n < 0 || r < 0)
    return 0;
  return fac[n] * (finv[r] * finv[n - r] % MOD) % MOD;
}

ll nCrcheep(ll n, ll r) {
  if (r == 0)
    return 1;
  else if (r == 1)
    return n;
  else
    return nCrcheep(n - 1, r - 1) * n / r;
}

vector<pair<ll, ll>> prime_factorize(ll n) {
  vector<pair<ll, ll>> res;

  for (ll i = 2; i * i <= n; i++) {
    if (n % i != 0)
      continue;

    ll ex = 0;
    while (n % i == 0) {
      ex++;
      n /= i;
    }

    res.push_back({i, ex});
  }

  if (n != 1)
    res.push_back({n, 1});

  return res;
}

int main() {
  ll sx, sy, tx, ty;
  cin >> sx >> sy >> tx >> ty;
  ll dx = tx - sx;
  ll dy = ty - sx;

  cout << string(dx, 'R') << string(dy, 'U') << string(dx, 'L')
       << string(dy, 'D') << 'D' << string(dx + 1, 'R') << string(dy + 1, 'U')
       << 'L' << 'U' << string(dx + 1, 'L') << string(dy + 1, 'D') << 'R'
       << endl;
}


Fixed submission:
#define _GLIBCXX_DEBUG
#include <algorithm> //next_permutation
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); i++)
#define all(v) v.begin(), v.end()
#define dec(n) cout << fixed << setprecision(n);
#define large "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
#define small "abcdefghijklmnopqrstuvwxyz"
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
using vl = vector<ll>;
using vvl = vector<vl>;

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

const ll MOD = 1000000007;
const ll MAX = 2000001;

ll mod(ll a) { return a % MOD; }

ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }

ll fac[MAX], finv[MAX], inv[MAX];

void nCrprep() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (ll i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

ll nCr(ll n, ll r) {
  if (n < r)
    return 0;
  if (n < 0 || r < 0)
    return 0;
  return fac[n] * (finv[r] * finv[n - r] % MOD) % MOD;
}

ll nCrcheep(ll n, ll r) {
  if (r == 0)
    return 1;
  else if (r == 1)
    return n;
  else
    return nCrcheep(n - 1, r - 1) * n / r;
}

vector<pair<ll, ll>> prime_factorize(ll n) {
  vector<pair<ll, ll>> res;

  for (ll i = 2; i * i <= n; i++) {
    if (n % i != 0)
      continue;

    ll ex = 0;
    while (n % i == 0) {
      ex++;
      n /= i;
    }

    res.push_back({i, ex});
  }

  if (n != 1)
    res.push_back({n, 1});

  return res;
}

int main() {
  ll sx, sy, tx, ty;
  cin >> sx >> sy >> tx >> ty;
  ll dx = tx - sx;
  ll dy = ty - sy;

  cout << string(dx, 'R') << string(dy, 'U') << string(dx, 'L')
       << string(dy, 'D') << 'D' << string(dx + 1, 'R') << string(dy + 1, 'U')
       << 'L' << 'U' << string(dx + 1, 'L') << string(dy + 1, 'D') << 'R'
       << endl;
}