#include <bits/stdc++.h>

using i32 = std::int32_t;
using u32 = std::uint32_t;
using i64 = std::int64_t;
using u64 = std::uint64_t;
using i128 = __int128_t;
using u128 = __uint128_t;
using isize = std::ptrdiff_t;
using usize = std::size_t;

class rep {
    struct Iter {
        usize itr;
        constexpr Iter(const usize pos) noexcept : itr(pos) {}
        constexpr void operator++() noexcept { ++itr; }
        constexpr bool operator!=(const Iter& other) const noexcept {
            return itr != other.itr;
        }
        constexpr usize operator*() const noexcept { return itr; }
    };
    const Iter first, last;

  public:
    explicit constexpr rep(const usize first, const usize last) noexcept
        : first(first), last(std::max(first, last)) {}
    constexpr Iter begin() const noexcept { return first; }
    constexpr Iter end() const noexcept { return last; }
};

template <class T> using Vec = std::vector<T>;

struct Frac {
    u64 p, q;
    Frac(const u64 x = 0, const u64 y = 1) : p(x), q(y) {
        const auto g = std::gcd(p, q);
        p /= g;
        q /= g;
    }
    Frac operator+(const Frac& other) const {
        return Frac(p * other.q + q * other.p, q * other.q);
    }
    Frac operator-(const Frac& other) const {
        return Frac(p * other.q - q * other.p, q * other.q);
    }
    Frac operator*(const Frac& other) const {
        return Frac(p * other.p, q * other.q);
    }
    Frac operator/(const Frac& other) const {
        return Frac(p * other.q, q * other.p);
    }
};

void C_main() {
    usize N, K;
    std::cin >> N >> K;
    if (N == 1) {
        std::cout << "1/1\n";
        return;
    }
    Vec<u32> A(N);
    for (auto& x : A) {
        std::cin >> x;
    }
    Vec<Frac> coeff(N);
    coeff[1] = Frac(1);
    for (const auto i : rep(2, N)) {
        coeff[i] = (coeff[i - 1] * (Frac(A[i]) + Frac(A[i - 2])) -
                    coeff[i - 2] * Frac(A[i - 2])) /
                   Frac(A[i]);
    }
    Vec<Frac> ans(N);
    for (const auto i : rep(0, N)) {
        ans[i] = coeff[i] / coeff[N - 1];
    }
    const auto [a, b] = ans[K - 1];
    if (a == 0) {
        std::cout << "0\n";
    } else {
        std::cout << a << "/" << b << '\n';
    }
}

int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);
    C_main();
    return 0;
}