Problem p02959 (C++)

Buggy submission (status: Runtime Error):
// Ruthless Coding
#include <bits/stdc++.h>

#define uni(x) (x).resize(unique(all(x)) - (x).begin())
#define fprint(v)                                                              \
  for (auto x : v)                                                             \
  cout << x << ' '
#define ALL(x) (x).begin(), (x).end()
#define MP(x, y) make_pair(x, y)
#define PII pair<int, int>
#define SZ(x) int((x).size())
#define PB(x) push_back(x)
#define ll long long
#define S second
#define F first
#define nl '\n'

using namespace std;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }

template <typename T, typename V> void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T> void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x)
    cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V> void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v))
    cerr << ", ";
  _print(v...);
}
#ifndef ONLINE_JUDGE
#define debug(x...)                                                            \
  cerr << "[" << #x << "] = [";                                                \
  _print(x)
#else
#define debug(x...)
#endif
mt19937_64 rnd;

const int N = 2e5 + 5;

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  vector<ll> a(n + 1), b(n);
  for (auto &x : a) {
    cin >> x;
  }
  for (auto &x : b) {
    cin >> x;
  }
  ll ans = min(a[0], b[0]);
  b[0] -= min(a[0], b[0]);
  for (int i = 1; i < n + 1; i++) {
    if (a[i] > 0) {
      if (b[i - 1] > 0) {
        ans += min(a[i], b[i - 1]);
        a[i] -= min(a[i], b[i - 1]);
      }
      if (b[i] > 0) {
        ans += min(a[i], b[i]);
        b[i] -= min(a[i], b[i]);
      }
    }
  }
  cout << ans;
  return 0;
}
/*
    *** Most Impo.. -> check base case always
    1. Overflow Check (*, +)
    2. Index check (0 - based or 1 - based)
    3. Check for n = 1, 2, 3, 4....
    4. Corner Cases
*/

Fixed submission:
// Ruthless Coding
#include <bits/stdc++.h>

#define uni(x) (x).resize(unique(all(x)) - (x).begin())
#define fprint(v)                                                              \
  for (auto x : v)                                                             \
  cout << x << ' '
#define ALL(x) (x).begin(), (x).end()
#define MP(x, y) make_pair(x, y)
#define PII pair<int, int>
#define SZ(x) int((x).size())
#define PB(x) push_back(x)
#define ll long long
#define S second
#define F first
#define nl '\n'

using namespace std;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }

template <typename T, typename V> void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T> void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x)
    cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V> void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v))
    cerr << ", ";
  _print(v...);
}
#ifndef ONLINE_JUDGE
#define debug(x...)                                                            \
  cerr << "[" << #x << "] = [";                                                \
  _print(x)
#else
#define debug(x...)
#endif
mt19937_64 rnd;

const int N = 2e5 + 5;

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  vector<ll> a(n + 1), b(n);
  for (auto &x : a) {
    cin >> x;
  }
  for (auto &x : b) {
    cin >> x;
  }
  ll ans = min(a[0], b[0]);
  b[0] -= min(a[0], b[0]);
  for (int i = 1; i < n + 1; i++) {
    if (a[i] > 0) {
      if (b[i - 1] > 0) {
        ans += min(a[i], b[i - 1]);
        a[i] -= min(a[i], b[i - 1]);
      }
      if (i < n && b[i] > 0) {
        ans += min(a[i], b[i]);
        b[i] -= min(a[i], b[i]);
      }
    }
  }
  cout << ans;
  return 0;
}
/*
    *** Most Impo.. -> check base case always
    1. Overflow Check (*, +)
    2. Index check (0 - based or 1 - based)
    3. Check for n = 1, 2, 3, 4....
    4. Corner Cases
*/