#include <vector>

long long numberOfDiscIntersections(const std::vector<int>& a)
{
    long long sum = 0;
    long long active = 0;
    int c = static_cast<int>(a.size());
    std::vector<int> start(c, 0);
    std::vector<int> end(c, 0);

    for (int k = 0; k < c; ++k) {
        long long v = a[k];

        long long key = (k < v) ? 0 : k - v;
        ++start[static_cast<int>(key)];

        long long key2 = (static_cast<long long>(k) + v >= c) ? c - 1 : k + v;
        ++end[static_cast<int>(key2)];
    }

    for (int k = 0; k < c; ++k) {
        sum += active * start[k] + (static_cast<long long>(start[k]) * (start[k] - 1)) / 2;
        active += start[k] - end[k];
        if (sum > 10000000) {
            return -1;
        }
    }

    return sum;
}