import sys
import math
from typing import List, Tuple
from fractions import Fraction

int1 = lambda x: int(x) - 1
input = lambda: sys.stdin.readline().rstrip('\n')
ii = lambda: int(input())
vi = lambda: list(map(int, input().split()))
vi1 = lambda: list(map(int1, input().split()))


def dbg(*args, **kwargs):
    print(*(repr(arg) for arg in args), *(f'{k}: {repr(v)}' for k, v in kwargs.items()),
          sep='; ', file=sys.stderr, flush=True)


def main():
    n = ii()
    x = vi()
    m = ii() - 1
    ex = Fraction(sum(x), n)
    ex2 = Fraction(sum(i * i for i in x), n)
    vx = ex2 - ex * ex
    if vx == 0:
        return 50
    d = x[m] - ex
    sig2_100 = (d * d * 10 * 10) / vx
    a, b = sig2_100.as_integer_ratio()
    if d >= 0:
        ans = 50 + math.isqrt(a * b) // b
    else:
        ans = 50 + (-(math.isqrt(a * b - 1) + 1)) // b
    return ans


def _start():
    if (ret := main()) is not None:
        print(*ret) if isinstance(ret, List) or isinstance(ret, Tuple) else print(ret)


if __name__ == '__main__':
    _start()