public class Solution {
    public static int numberOfDiscIntersections(int[] a) {
        int c = a.length;
        long sum = 0;
        long active = 0;
        int[] start = new int[c];
        int[] end = new int[c];

        for (int k = 0; k < c; k++) {
            long v = a[k];
            int key = (k < v) ? 0 : (int) (k - v);
            start[key]++;

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

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

        return (int) sum;
    }
}