#include <boost/integer/common_factor.hpp>
#include <boost/multiprecision/cpp_int.hpp>

#include <array>
#include <cstdint>
#include <iostream>
#include <numeric>
#include <queue>

using u64 = boost::multiprecision::cpp_int;

struct rational {
  u64 num;
  u64 den;

  rational() : num(0), den(1) {}

  rational(u64 n, u64 d) : num(n), den(d) {
    u64 g = boost::integer::gcd(n, d);
    num /= g;
    den /= g;
  }

  friend rational operator+(const rational &l, const rational &r) {
    return {l.num * r.den + r.num * l.den, l.den * r.den};
  }

  friend rational operator-(const rational &l, const rational &r) {
    return {l.num * r.den - r.num * l.den, l.den * r.den};
  }

  friend rational operator*(const rational &l, const rational &r) {
    return {l.num * r.num, l.den * r.den};
  }

  friend rational operator/(const rational &l, const rational &r) {
    return {l.num * r.den, l.den * r.num};
  }
};

using gate = std::array<std::array<rational, 2>, 2>;

gate concat(const gate &l, const gate &r) {
  gate res = {};
  res[0][1] =
      (rational(1, 1) - l[0][0]) * r[0][1] / (r[0][0] * l[1][0] + r[0][1]);
  res[0][0] = rational(1, 1) - res[0][1];
  res[1][0] =
      (rational(1, 1) - r[1][1]) * l[1][0] / (l[1][1] * r[0][1] + l[1][0]);
  res[1][1] = rational(1, 1) - res[1][0];
  return res;
}

int main() {
  int n, k;
  std::cin >> n >> k;
  if (n == k) {
    std::cout << "1/1\n";
    return 0;
  }
  if (k == 1) {
    std::cout << "0\n";
    return 0;
  }
  k -= 2;

  std::vector<int> a(n);
  for (auto &e : a) {
    std::cin >> e;
  }
  std::vector<gate> g(n - 2);
  for (int i = 0; i != n - 2; i += 1) {
    g[i][0][0] = g[i][1][0] = rational(a[i], a[i] + a[i + 2]);
    g[i][0][1] = g[i][1][1] = rational(a[i + 2], a[i] + a[i + 2]);
  }

  const auto aggregate = [&](const int l, const int r) {
    std::queue<gate> que;
    {
      gate t;
      t[0][0] = t[1][1] = rational(0, 1);
      t[0][1] = t[1][0] = rational(1, 1);
      que.push(t);
    }
    for (int i = l; i != r; i += 1) {
      que.push(g[i]);
    }
    while (que.size() != 1) {
      std::queue<gate> next;
      while (que.size() >= 2) {
        gate x = que.front();
        que.pop();
        gate y = que.front();
        que.pop();
        next.push(concat(x, y));
      }
      if (!que.empty()) {
        next.push(que.front());
      }
      que = std::move(next);
    }
    return que.front();
  };

  gate l = aggregate(0, k);
  gate r = aggregate(k, n - 2);

  rational ans = r[0][1] / (r[0][1] + r[0][0] * l[1][0]);

  std::cout << ans.num << "/" << ans.den << "\n";

  return 0;
}