func numberOfDiscIntersections(a []int) int {
	c := len(a)
	start := make([]int, c)
	end := make([]int, c)

	for k, v := range a {
		key := k - v
		if k < v {
			key = 0
		}
		start[key]++

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

	sum, active := 0, 0
	for k := range a {
		sum += active*start[k] + (start[k]*(start[k]-1))/2
		active += start[k] - end[k]
		if sum > 10000000 {
			return -1
		}
	}

	return sum
}