CF466C

// Problem: C. Number of Ways
// Contest: Codeforces - Codeforces Round 266 (Div. 2)
// URL: https://codeforces.com/problemset/problem/466/C
// Memory Limit: 256 MB
// Time Limit: 2000 ms
//
// Powered by CP Editor (https://cpeditor.org)

#include <assert.h>
#include <bits/stdc++.h>
using namespace std;
#ifndef __DEBUG__
#define dbg(...) 42
#endif
template <class T> using mpq = priority_queue<T, vector<T>, greater<T>>;

using ll = long long;
using a2l = array<ll, 2>;
using pll = pair<ll, ll>;
using vl = vector<ll>;

void solve()
{
    ll n;
    cin >> n;
    vl a(n);
    for (auto &x : a)
        cin >> x;
    ll tot = accumulate(a.begin(), a.end(), 0ll), ans = 0;
    if (tot % 3) {
        cout << 0 << '\n';
        return;
    }

    vl post1(n), post2(n);
    for (ll i = n - 1, sum = 0; i >= 0; --i) {
        sum += a[i];
        post1[i] = (i == n - 1 ? 0 : post1[i + 1]) + (sum == tot / 3);
    }
    for (ll i = n - 1, sum = 0; i >= 0; --i) {
        sum += a[i];
        post2[i] = (sum == tot / 3 * 2 ? (i == n - 1 ? 0 : post1[i + 1]) : 0);
    }

    for (ll i = 0, sum = 0; i < n; ++i) {
        sum += a[i];
        if (sum == tot / 3)
            ans += i + 1 < n ? post2[i + 1] : 0;
    }
    dbg(post1, post2);
    cout << ans << '\n';
}

int main(int argc, char **argv)
{
    solve();
    return 0;
};