fn number_of_disc_intersections(a: &[i64]) -> i64 {
    let c = a.len();
    let mut start = vec![0i64; c];
    let mut end = vec![0i64; c];

    for (k, &v) in a.iter().enumerate() {
        let k = k as i64;
        let key = if k < v { 0 } else { (k - v) as usize };
        start[key] += 1;

        let key = if k + v >= c as i64 { c - 1 } else { (k + v) as usize };
        end[key] += 1;
    }

    let mut sum = 0i64;
    let mut active = 0i64;
    for k in 0..c {
        sum += active * start[k] + (start[k] * (start[k] - 1)) / 2;
        active += start[k] - end[k];
        if sum > 10_000_000 {
            return -1;
        }
    }

    sum
}