M - Friendship

#include <bits/stdc++.h>
#define fi first
#define se second
using namespace std;

typedef long long i64;

const int MAXN = 500010;

int n;
int ft[MAXN];

void add(int idx, int val) {
    for (; idx <= n; idx += (idx & -idx)) {
        ft[idx] += val;
    }
}

int get(int idx) {
    int ans = 0;
    for (; idx > 0; idx -= (idx & -idx)) {
        ans += ft[idx];
    }
    return ans;
}

void add_range(int l, int r, int val) {
    l = max(1, l); r = min(r, n);
    add(l, val);
    add(r + 1, -val);
}

int main() {
  cin.tie(0)->sync_with_stdio(0);

  cin >> n;

  vector<int> a(n + 1), b(n + 1);

    vector<pair<int, int>> ranges;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
        if (a[i])
            ranges.push_back({ max(1, i - a[i]), i });
  }

    sort(ranges.begin(), ranges.end());

  for (int i = 1; i <= n; i++) {
    cin >> b[i];
        add_range(i + 1, i + b[i], 1);
  }

    i64 ans = 0;
    int curr = 1;

    for (int i = 0; i < (int)ranges.size(); ++i) {
        while (curr <= n and curr < ranges[i].fi) {
            add_range(curr + 1, curr + b[curr], -1);
            ++curr;
        }
        ans += get(ranges[i].se);
    }

    cout << ans << '\n';
}