int Interpol(int[] arr, int elem) {
    int l = 0, r = arr.Length - 1;
    int m;
    while (arr[l] <= elem && arr[r] >= elem) {
        m = Convert.ToInt32( l + ((1.0 * r - l) * (elem - arr[l])) / (arr[r] - arr[l]));
        if (arr[m] < elem) l = m + 1;
        else if (arr[m] > elem) r = m - 1;
        else return m;
    }
    if (arr[l] == elem) return l;
    else return -1;

}