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#include <cassert>
#include <iostream>
#include <vector>

using namespace std;
using ll = long long;
#define all(a) begin(a), end(a)

int rand(int l, int r) { return rand() % (r - l + 1) + l; }

void solve() {
    int n = rand(1, 10);
    cin >> n;
    vector<ll> a(n);
    for (auto &i : a) {
        // i = rand(1, 10);
        cin >> i;
    }

    a.push_back(0);
    vector<ll> d(n + 1);
    ll prev = 0;
    for (int i = 0; i < n + 1; i++) {
        d[i] = a[i] - prev;
        // cerr << d[i] << " ";
        prev = a[i];
    }
    // cerr << "\n";

    auto check = [&](int sz) {
        auto t = d;
        for (int j = 0; sz + j <= n; j++) {
            ll f = t[j];
            if (f < 0) {
                return false;
            }
            t[sz + j] += f;
            t[j] = 0;
        }

        for (int j = n - sz + 1; j <= n; j++) {
            if (t[j] != 0) {
                return false;
            }
        }
        return true;
    };

    vector<int> pr;
    for (int i = 2; i <= n; i++) {
        bool p = 1;
        for (int j = 2; j * j <= i; j++) {
            p &= i % j != 0;
        }

        if (!p) {
            continue;
        }

        if (check(i)) {
            pr.push_back(i);
        }
    }

    for (int i = n; i >= 1; i--) {
        int t = i;
        for (auto j : pr) {
            while (t % j == 0) {
                t /= j;
            }
        }
        if (t != 1) {
            continue;
        }

        if (check(i)) {
            cout << i << "\n";
            return;
        }
    }
    assert(false);
}

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    int q = 1;
    // cin >> q;
    while (q--) {
        solve();
    }
}