Problem p02959 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep3(i, l, n) for (int i = l; i < (n); ++i)
#define sz(v) (int)v.size()
const int inf = 1e9 + 7;
const ll INF = 1e18;
int mod;
#define abs(x) (x >= 0 ? x : -(x))
#define lb(v, x) (int)(lower_bound(all(v), x) - v.begin())
#define ub(v, x) (int)(upper_bound(all(v), x) - v.begin())
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> T gcd(T a, T b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
template <typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
template <typename T> T pow(T a, int b) {
  return b ? pow(a * a, b / 2) * (b % 2 ? a : 1) : 1;
}
ll modpow(ll a, ll b, ll _mod) {
  return b ? modpow(a * a % _mod, b / 2, _mod) * (b % 2 ? a : 1) % _mod : 1;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (auto &vi : vec)
    os << vi << " ";
  return os;
}
template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << p.F << " " << p.S;
  return os;
}
template <typename T> inline istream &operator>>(istream &is, vector<T> &v) {
  rep(j, sz(v)) is >> v[j];
  return is;
}
template <class T> inline void add(T &a, int b) {
  a += b;
  if (a >= mod)
    a -= mod;
}

void solve();

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cout << fixed << setprecision(10);

  int T;
  // cin >> T;
  T = 1;

  while (T--) {
    solve();
  }
}

template <typename T> vector<T> lis(vector<T> a) {
  vector<T> b;
  b.eb(a[0]);
  rep3(i, 1, sz(a)) {
    int it = lb(b, a[i]);
    if (it == sz(b)) {
      b.eb(a[i]);
    } else {
      b[it] = a[i];
    }
  }
  return b;
}

void solve() {
  int n;
  cin >> n;
  vector<ll> a(n + 1);
  cin >> a;
  ll A = accumulate(all(a), 0LL);
  vector<ll> b(n);
  cin >> b;

  rep(i, n + 1) {
    ll c;
    if (i) {
      c = min(a[i], b[i - 1]);
      a[i] -= c;
    }
    c = min(a[i], b[i]);
    a[i] -= c;
    b[i] -= c;
  }
  ll B = accumulate(all(a), 0LL);
  cout << A - B << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep3(i, l, n) for (int i = l; i < (n); ++i)
#define sz(v) (int)v.size()
const int inf = 1e9 + 7;
const ll INF = 1e18;
int mod;
#define abs(x) (x >= 0 ? x : -(x))
#define lb(v, x) (int)(lower_bound(all(v), x) - v.begin())
#define ub(v, x) (int)(upper_bound(all(v), x) - v.begin())
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> T gcd(T a, T b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
template <typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
template <typename T> T pow(T a, int b) {
  return b ? pow(a * a, b / 2) * (b % 2 ? a : 1) : 1;
}
ll modpow(ll a, ll b, ll _mod) {
  return b ? modpow(a * a % _mod, b / 2, _mod) * (b % 2 ? a : 1) % _mod : 1;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (auto &vi : vec)
    os << vi << " ";
  return os;
}
template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << p.F << " " << p.S;
  return os;
}
template <typename T> inline istream &operator>>(istream &is, vector<T> &v) {
  rep(j, sz(v)) is >> v[j];
  return is;
}
template <class T> inline void add(T &a, int b) {
  a += b;
  if (a >= mod)
    a -= mod;
}

void solve();

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cout << fixed << setprecision(10);

  int T;
  // cin >> T;
  T = 1;

  while (T--) {
    solve();
  }
}

template <typename T> vector<T> lis(vector<T> a) {
  vector<T> b;
  b.eb(a[0]);
  rep3(i, 1, sz(a)) {
    int it = lb(b, a[i]);
    if (it == sz(b)) {
      b.eb(a[i]);
    } else {
      b[it] = a[i];
    }
  }
  return b;
}

void solve() {
  int n;
  cin >> n;
  vector<ll> a(n + 1);
  cin >> a;
  ll A = accumulate(all(a), 0LL);
  vector<ll> b(n);
  cin >> b;

  rep(i, n + 1) {
    ll c;
    if (i) {
      c = min(a[i], b[i - 1]);
      a[i] -= c;
    }
    if (i != n) {
      c = min(a[i], b[i]);
      a[i] -= c;
      b[i] -= c;
    }
  }
  ll B = accumulate(all(a), 0LL);
  cout << A - B << endl;
}