#!/usr/bin/env python3
import fractions
from typing import *


def solve(n: int, k: int, x: List[int]) -> str:
    if k == n:
        return 1
    a = [None] * (n - 1)
    b = [None] * (n - 1)
    a[0] = 0
    b[0] = 0
    for i in range(1, n - 1):
        left = fractions.Fraction(x[i - 1], x[i - 1] + x[i + 1])
        right = fractions.Fraction(x[i + 1], x[i - 1] + x[i + 1])
        a[i] = right / (1 - a[i - 1] * left)
        b[i] = b[i - 1] * left / (1 - a[i - 1] * left)
    c = 1
    for i in reversed(range(k - 1, n - 1)):
        c = a[i] * c + b[i]
    return c


# generated by oj-template v4.7.2 (https://github.com/online-judge-tools/template-generator)
def main():
    import sys
    tokens = iter(sys.stdin.read().split())
    N = int(next(tokens))
    K = int(next(tokens))
    A = [None for _ in range(N)]
    for i in range(N):
        A[i] = int(next(tokens))
    assert next(tokens, None) is None
    a = solve(N, K, A)
    if a == 0:
        print(0)
    elif a == 1:
        print('1/1')
    else:
        print(a)


if __name__ == '__main__':
    main()