#include <bits/stdc++.h>
using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n;
    if (!(cin >> n)) return 0;
    vector<long long> s(n);
    for (int i = 0; i < n; ++i) cin >> s[i];

    long long ans = 0;

    // 最初の for i in range(n)
    for (int i = 0; i < n; ++i) {
        int left = 0;
        int right = min(i, n - i - 1) + 1;
        while (right > left + 1) {
            int mid = (left + right) / 2;
            bool p = true;
            int start = max(mid - 10, 0);
            for (int j = start; j <= mid; ++j) {
                if (s[i + j] + s[i - j] != s[i] * 2) {
                    p = false;
                    break; // Python の元ループは break しないが、ここで早期脱出しても結果は同じです
                }
            }
            if (p) left = mid;
            else right = mid;
        }
        ans += left;
    }

    // 次の for i in range(1, n)
    for (int i = 1; i < n; ++i) {
        int left = 1;
        int right = min(i, n - i) + 1;
        while (right > left + 1) {
            int mid = (left + right) / 2;
            bool p = true;
            int start = max(mid - 10, 1);
            for (int j = start; j <= mid; ++j) {
                if (s[i - j] + s[i + j - 1] != s[i - 1] + s[i]) {
                    p = false;
                    break;
                }
            }
            if (p) left = mid;
            else right = mid;
        }
        ans += left;
    }

    cout << (ans + n) << '\n';
    return 0;
}