function numberOfDiscIntersections(a: number[]): number {
  let sum = 0;
  let active = 0;
  const c = a.length;
  const start: number[] = new Array(c).fill(0);
  const end: number[] = new Array(c).fill(0);

  for (let k = 0; k < c; k++) {
    const v = a[k];
    const startKey = k < v ? 0 : k - v;
    start[startKey]++;

    const endKey = k + v >= c ? c - 1 : k + v;
    end[endKey]++;
  }

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

  return sum;
}