number_of_disc_intersections() {
    local -n _a="$1"
    local _c=${#_a[@]}
    local -a _start _end
    local _k
    for ((_k = 0; _k < _c; _k++)); do _start[_k]=0; _end[_k]=0; done
    for _k in "${!_a[@]}"; do
        local _v=${_a[$_k]}
        local _key
        if (( _k < _v )); then _key=0; else _key=$(( _k - _v )); fi
        (( _start[_key]++ ))
        if (( _k + _v >= _c )); then _key=$(( _c - 1 )); else _key=$(( _k + _v )); fi
        (( _end[_key]++ ))
    done
    local _sum=0 _active=0
    for ((_k = 0; _k < _c; _k++)); do
        _sum=$(( _sum + _active * _start[_k] + (_start[_k] * (_start[_k] - 1)) / 2 ))
        _active=$(( _active + _start[_k] - _end[_k] ))
        if (( _sum > 10000000 )); then
            echo -1
            return
        fi
    done
    echo "$_sum"
}