Language: C++

#include <iostream>
#include <vector>
#include <algorithm>
#include <utility>
#include <queue>
#include <set>
#include <map>
#define all(x) (x).begin(), (x).end()
typedef long long ll;
#define MOD 1000000007
using namespace std;
//https://ei1333.github.io/luzhiled/snippets/structure/segment-tree.html
template <typename Monoid>
struct SegmentTree
{
    using F = function<Monoid(Monoid, Monoid)>;

    int sz;
    vector<Monoid> seg;

    const F f;
    const Monoid M1;

    SegmentTree(int n, const F f, const Monoid &M1) : f(f), M1(M1)
    {
        sz = 1;
        while (sz < n)
            sz <<= 1;
        seg.assign(2 * sz, M1);
    }

    void set(int k, const Monoid &x)
    {
        seg[k + sz] = x;
    }

    void build()
    {
        for (int k = sz - 1; k > 0; k--)
        {
            seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
        }
    }

    void update(int k, const Monoid &x)
    {
        k += sz;
        seg[k] = x;
        while (k >>= 1)
        {
            seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
        }
    }

    Monoid query(int a, int b)
    {
        Monoid L = M1, R = M1;
        for (a += sz, b += sz; a < b; a >>= 1, b >>= 1)
        {
            if (a & 1)
                L = f(L, seg[a++]);
            if (b & 1)
                R = f(seg[--b], R);
        }
        return f(L, R);
    }

    Monoid operator[](const int &k) const
    {
        return seg[k + sz];
    }

    template <typename C>
    int find_subtree(int a, const C &check, Monoid &M, bool type)
    {
        while (a < sz)
        {
            Monoid nxt = type ? f(seg[2 * a + type], M) : f(M, seg[2 * a + type]);
            if (check(nxt))
                a = 2 * a + type;
            else
                M = nxt, a = 2 * a + 1 - type;
        }
        return a - sz;
    }

    template <typename C>
    int find_first(int a, const C &check)
    {
        Monoid L = M1;
        if (a <= 0)
        {
            if (check(f(L, seg[1])))
                return find_subtree(1, check, L, false);
            return -1;
        }
        int b = sz;
        for (a += sz, b += sz; a < b; a >>= 1, b >>= 1)
        {
            if (a & 1)
            {
                Monoid nxt = f(L, seg[a]);
                if (check(nxt))
                    return find_subtree(a, check, L, false);
                L = nxt;
                ++a;
            }
        }
        return -1;
    }

    template <typename C>
    int find_last(int b, const C &check)
    {
        Monoid R = M1;
        if (b >= sz)
        {
            if (check(f(seg[1], R)))
                return find_subtree(1, check, R, true);
            return -1;
        }
        int a = sz;
        for (b += sz; a < b; a >>= 1, b >>= 1)
        {
            if (b & 1)
            {
                Monoid nxt = f(seg[--b], R);
                if (check(nxt))
                    return find_subtree(b, check, R, true);
                R = nxt;
            }
        }
        return -1;
    }
};
int main()
{
    ll n, k;
    cin >> n >> k;
    vector<ll> a(n);
    for (size_t i = 0; i < n; i++)
    {
        cin >> a[i];
    }
    vector<ll> r(n + 1, 0), p(n + 1, 0);
    for (size_t i = 0; i < n; i++)
    {
        r[i + 1] = r[i] + a[i];
        p[i + 1] = r[i + 1] - (i + 1) * k;
    }
    vector<ll> b = p;
    map<ll, ll> mp;
    sort(all(b));
    for (int i = 0; i < n + 1; i++)
    {
        mp[b[i]] = i + 1;
    }

    ll ans = 0;
    SegmentTree<int> seg(n + 1, [](int a, int b) { return a + b; }, 0);
    seg.build();
    for (int i = 0; i < n + 1; i++)
    {
        ans += seg.query(1, i + 1);
        seg.update(mp[p[i]], 1);
    }

    cout << ans << endl;
}

Compiler output:
a.cc:16:15: error: 'function' does not name a type; did you mean 'union'?
   16 |     using F = function<Monoid(Monoid, Monoid)>;
      |               ^~~~~~~~
      |               union
a.cc:21:11: error: 'F' does not name a type
   21 |     const F f;
      |           ^
a.cc:24:30: error: 'F' does not name a type
   24 |     SegmentTree(int n, const F f, const Monoid &M1) : f(f), M1(M1)
      |                              ^
a.cc: In constructor 'SegmentTree<Monoid>::SegmentTree(int, int, const Monoid&)':
a.cc:24:55: error: class 'SegmentTree<Monoid>' does not have any field named 'f'
   24 |     SegmentTree(int n, const F f, const Monoid &M1) : f(f), M1(M1)
      |                                                       ^
a.cc: In function 'int main()':
a.cc:160:33: error: invalid user-defined conversion from 'main()::<lambda(int, int)>' to 'int' [-fpermissive]
  160 |     SegmentTree<int> seg(n + 1, [](int a, int b) { return a + b; }, 0);
      |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:160:33: note: candidate is: 'constexpr main()::<lambda(int, int)>::operator int (*)(int, int)() const' (near match)
  160 |     SegmentTree<int> seg(n + 1, [](int a, int b) { return a + b; }, 0);
      |                                 ^
a.cc:160:33: note:   no known conversion from 'int (*)(int, int)' to 'int'
a.cc:24:32: note:   initializing argument 2 of 'SegmentTree<Monoid>::SegmentTree(int, int, const Monoid&) [with Monoid = int]'
   24 |     SegmentTree(int n, const F f, const Monoid &M1) : f(f), M1(M1)
      |                        ~~~~~~~~^
a.cc: In instantiation of 'void SegmentTree<Monoid>::build() [with Monoid = int]':
a.cc:161:14:   required from here
  161 |     seg.build();
      |     ~~~~~~~~~^~
a.cc:41:23: error: 'f' was not declared in this scope
   41 |             seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
      |                      ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc: In instantiation of 'Monoid SegmentTree<Monoid>::query(int, int) [with Monoid = int]':
a.cc:164:25:   required from here
  164 |         ans += seg.query(1, i + 1);
      |                ~~~~~~~~~^~~~~~~~~~
a.cc:61:22: error: 'f' was not declared in this scope
   61 |                 L = f(L, seg[a++]);
      |                     ~^~~~~~~~~~~~~
a.cc:63:22: error: 'f' was not declared in this scope
   63 |                 R = f(seg[--b], R);
      |                     ~^~~~~~~~~~~~~
a.cc:65:17: error: 'f' was not declared in this scope
   65 |         return f(L, R);
      |                ~^~~~~~
a.cc: In instantiation of 'void SegmentTree<Monoid>::update(int, const Monoid&) [with Monoid = int]':
a.cc:165:19:   required from here
  165 |         seg.update(mp[p[i]], 1);
      |         ~~~~~~~~~~^~~~~~~~~~~~~
a.cc:51:23: error: 'f' was not declared in this scope
   51 |             seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
      |                      ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~