Problem p03347 (C++)

Buggy submission (status: Runtime Error):
#include "bits/stdc++.h"
#include <random>
using namespace std;
typedef long long int lint;
typedef pair<lint, lint> plint;
typedef pair<double long, double long> pld;
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((lint)(x).size())
#define POW2(n) (1LL << (n))
#define FOR(i, begin, end)                                                     \
  for (lint i = (begin), i##_end_ = (end); i < i##_end_; i++)
#define IFOR(i, begin, end)                                                    \
  for (lint i = (end)-1, i##_begin_ = (begin); i >= i##_begin_; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T1, typename T2>
pair<T1, T2> operator+(const pair<T1, T2> &l, const pair<T1, T2> &r) {
  return make_pair(l.first + r.first, l.second + r.second);
}
template <typename T1, typename T2>
pair<T1, T2> operator-(const pair<T1, T2> &l, const pair<T1, T2> &r) {
  return make_pair(l.first - r.first, l.second - r.second);
}
const lint MOD = 1e9 + 7, INF = 1e18;

lint N, arr[100001];
int main() {
  cin >> N;
  REP(i, N) { cin >> arr[i]; }
  lint prev = -1;
  REP(i, N) {
    if (arr[i] > prev + 1) {
      cout << -1 << endl;
      return 0;
    }
    prev = arr[i];
  }
  arr[N] = 0;
  lint cnt = 0;
  REP(i, N) {
    if (arr[i] + 1 != arr[i + 1]) {
      cnt += arr[i];
    }
  }
  cout << cnt << endl;
}


Fixed submission:
#include "bits/stdc++.h"
#include <random>
using namespace std;
typedef long long int lint;
typedef pair<lint, lint> plint;
typedef pair<double long, double long> pld;
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((lint)(x).size())
#define POW2(n) (1LL << (n))
#define FOR(i, begin, end)                                                     \
  for (lint i = (begin), i##_end_ = (end); i < i##_end_; i++)
#define IFOR(i, begin, end)                                                    \
  for (lint i = (end)-1, i##_begin_ = (begin); i >= i##_begin_; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T1, typename T2>
pair<T1, T2> operator+(const pair<T1, T2> &l, const pair<T1, T2> &r) {
  return make_pair(l.first + r.first, l.second + r.second);
}
template <typename T1, typename T2>
pair<T1, T2> operator-(const pair<T1, T2> &l, const pair<T1, T2> &r) {
  return make_pair(l.first - r.first, l.second - r.second);
}
const lint MOD = 1e9 + 7, INF = 1e18;

lint N, arr[200001];
int main() {
  cin >> N;
  REP(i, N) { cin >> arr[i]; }
  lint prev = -1;
  REP(i, N) {
    if (arr[i] > prev + 1) {
      cout << -1 << endl;
      return 0;
    }
    prev = arr[i];
  }
  arr[N] = 0;
  lint cnt = 0;
  REP(i, N) {
    if (arr[i] + 1 != arr[i + 1]) {
      cnt += arr[i];
    }
  }
  cout << cnt << endl;
}