Problem p02778 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define fastcin()                                                              \
  cin.tie(0);                                                                  \
  ios::sync_with_stdio(false)
#define rep(i, n) for (int i = 0; i < int(n); i++)
#define rep1(i, n) for (int i = 1; i <= int(n); i++)
#define per(i, n) for (int i = int(n) - 1; i >= 0; i--)
#define per1(i, n) for (int i = int(n); i > 0; i--)
#define foreach(i, n) for (auto &&i : n)
#define all(x) (x).begin(), (x).end()
#define SORT(x) sort(all(x))
#define REV(x) reverse(all(x))
#define MAX(x) *max_element(all(x))
#define MIN(x) *min_element(all(x))
#define LOWitr(x, n) lower_bound(all(x), n)
#define UPRitr(x, n) upper_bound(all(x), n)
#define cii(x)                                                                 \
  int x;                                                                       \
  cin >> x
#define cill(x)                                                                \
  LL x;                                                                        \
  cin >> x
#define cis(x)                                                                 \
  string x;                                                                    \
  cin >> x
#define co(x) cout << x << endl
#define dump(x) cout << #x << " = " << (x) << endl
#define truecheck assert
#define pb push_back
#define mp make_pair
#define F first
#define S second
using namespace std;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef long long int64;
template <class T, class U> bool chmax(T &a, const U &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T, class U> bool chmin(T &a, const U &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}
const int INF = 1e9, MOD = 1e9 + 7;
const LL LLINF = 1e16;

int main() {
  fastcin();
  cout << fixed << setprecision(10);
  cis(s);
  auto ans = "";
  rep(i, s.length()) { ans += 'x'; }
  co(ans);
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#define fastcin()                                                              \
  cin.tie(0);                                                                  \
  ios::sync_with_stdio(false)
#define rep(i, n) for (int i = 0; i < int(n); i++)
#define rep1(i, n) for (int i = 1; i <= int(n); i++)
#define per(i, n) for (int i = int(n) - 1; i >= 0; i--)
#define per1(i, n) for (int i = int(n); i > 0; i--)
#define foreach(i, n) for (auto &&i : n)
#define all(x) (x).begin(), (x).end()
#define SORT(x) sort(all(x))
#define REV(x) reverse(all(x))
#define MAX(x) *max_element(all(x))
#define MIN(x) *min_element(all(x))
#define LOWitr(x, n) lower_bound(all(x), n)
#define UPRitr(x, n) upper_bound(all(x), n)
#define cii(x)                                                                 \
  int x;                                                                       \
  cin >> x
#define cill(x)                                                                \
  LL x;                                                                        \
  cin >> x
#define cis(x)                                                                 \
  string x;                                                                    \
  cin >> x
#define co(x) cout << x << endl
#define dump(x) cout << #x << " = " << (x) << endl
#define truecheck assert
#define pb push_back
#define mp make_pair
#define F first
#define S second
using namespace std;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef long long int64;
template <class T, class U> bool chmax(T &a, const U &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T, class U> bool chmin(T &a, const U &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}
const int INF = 1e9, MOD = 1e9 + 7;
const LL LLINF = 1e16;

int main() {
  fastcin();
  cout << fixed << setprecision(10);
  cis(s);
  string ans = "";
  rep(i, s.length()) { ans += "x"; }
  co(ans);
  return 0;
}