def number_of_disc_intersections(a: list[int]) -> int:
    c = len(a)
    start = [0] * c
    end = [0] * c

    for k, v in enumerate(a):
        key = 0 if k < v else k - v
        start[key] += 1

        key = c - 1 if k + v >= c else k + v
        end[key] += 1

    total = 0
    active = 0
    for k in range(c):
        total += active * start[k] + (start[k] * (start[k] - 1)) // 2
        active += start[k] - end[k]
        if total > 10000000:
            return -1

    return total