// >>> TEMPLATES
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using i32 = int32_t;
using i64 = int64_t;
using u32 = uint32_t;
using u64 = uint64_t;
#define int ll
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
#define repR(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep1R(i, n) for (int i = (int)(n); i >= 1; i--)
#define loop(i, a, B) for (int i = a; i B; i++)
#define loopR(i, a, B) for (int i = a; i B; i--)
#define all(x) begin(x), end(x)
#define allR(x) rbegin(x), rend(x)
#define rng(x, l, r) begin(x) + (l), begin(x) + (r)
#define pb push_back
#define eb emplace_back
#define fst first
#define snd second
template <class A, class B> constexpr auto mp(A &&a, B &&b) { return make_pair(forward<A>(a), forward<B>(b)); }
template <class... T> constexpr auto mt(T&&... x) { return make_tuple(forward<T>(x)...); }
template <class Int> auto constexpr inf_ = numeric_limits<Int>::max()/2-1;
auto constexpr INF32 = inf_<int32_t>;
auto constexpr INF64 = inf_<int64_t>;
auto constexpr INF   = inf_<int>;
#ifdef LOCAL
#include "debug.hpp"
#else
#define dump(...) (void)(0)
#define say(x) (void)(0)
#define debug if (0)
#endif
template <class T, class Comp> struct pque : priority_queue<T, vector<T>, Comp> { vector<T> &data() { return this->c; } void clear() { this->c.clear(); } };
template <class T> using pque_max = pque<T, less<T>>;
template <class T> using pque_min = pque<T, greater<T>>;
template <class T, class = typename T::iterator, enable_if_t<!is_same<T, string>::value, int> = 0>
ostream& operator<<(ostream& os, T const& a) { bool f = true; for (auto const& x : a) os << (f ? "" : " ") << x, f = false; return os; }
template <class T, size_t N, enable_if_t<!is_same<T, char>::value, int> = 0>
ostream& operator<<(ostream& os, const T (&a)[N]) { bool f = true; for (auto const& x : a) os << (f ? "" : " ") << x, f = false; return os; }
template <class T, class = decltype(begin(declval<T&>())), class = typename enable_if<!is_same<T, string>::value>::type>
istream& operator>>(istream& is, T &a) { for (auto& x : a) is >> x; return is; }
template <class T, class S> ostream& operator<<(ostream& os, pair<T, S> const& p) { return os << p.first << " " << p.second; }
template <class T, class S> istream& operator>>(istream& is, pair<T, S>& p) { return is >> p.first >> p.second; }
struct IOSetup { IOSetup() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } } iosetup;
template <class F> struct FixPoint : private F {
    constexpr FixPoint(F&& f) : F(forward<F>(f)) {}
    template <class... T> constexpr auto operator()(T&&... x) const { return F::operator()(*this, forward<T>(x)...); }
};
struct MakeFixPoint { template <class F> constexpr auto operator|(F&& f) const { return FixPoint<F>(forward<F>(f)); } };
#define MFP MakeFixPoint()|
#define def(name, ...) auto name = MFP [&](auto &&name, __VA_ARGS__)
template <class T, size_t d> struct vec_impl {
    using type = vector<typename vec_impl<T, d-1>::type>;
    template <class... U> static type make_v(size_t n, U&&... x) { return type(n, vec_impl<T, d-1>::make_v(forward<U>(x)...)); }
};
template <class T> struct vec_impl<T, 0> { using type = T; static type make_v(T const& x = {}) { return x; } };
template <class T, size_t d = 1> using vec = typename vec_impl<T, d>::type;
template <class T, size_t d = 1, class... Args> auto make_v(Args&&... args) { return vec_impl<T, d>::make_v(forward<Args>(args)...); }
template <class T> void quit(T const& x) { cout << x << endl; exit(0); }
template <class T, class U> constexpr bool chmin(T& x, U const& y) { if (x > y) { x = y; return true; } return false; }
template <class T, class U> constexpr bool chmax(T& x, U const& y) { if (x < y) { x = y; return true; } return false; }
template <class It> constexpr auto sumof(It b, It e) { return accumulate(b, e, typename iterator_traits<It>::value_type{}); }
template <class T> int sz(T const& x) { return x.size(); }
template <class C, class T> int lbd(C const& v, T const& x) { return lower_bound(begin(v), end(v), x)-begin(v); }
template <class C, class T> int ubd(C const& v, T const& x) { return upper_bound(begin(v), end(v), x)-begin(v); }
constexpr int64_t mod(int64_t x, int64_t m) { assert(m > 0); return (x %= m) < 0 ? x+m : x; }
constexpr int64_t div_floor(int64_t x, int64_t y) { assert(y != 0); return x/y - ((x^y) < 0 and x%y); }
constexpr int64_t div_ceil(int64_t x, int64_t y) { assert(y != 0); return x/y + ((x^y) > 0 and x%y); }
constexpr int dx[] = { 1, 0, -1, 0, 1, -1, -1, 1 };
constexpr int dy[] = { 0, 1, 0, -1, 1, 1, -1, -1 };
constexpr int popcnt(ll x) { return __builtin_popcountll(x); }
mt19937_64 seed_{random_device{}()};
template <class Int> Int rand(Int a, Int b) { return uniform_int_distribution<Int>(a, b)(seed_); }
i64 irand(i64 a, i64 b) { return rand<i64>(a, b); } // [a, b]
u64 urand(u64 a, u64 b) { return rand<u64>(a, b); } //
template <class It> void shuffle(It l, It r) { shuffle(l, r, seed_); }
vector<int> &operator--(vector<int> &v) { for (int &x : v) --x; return v; }
vector<int> &operator++(vector<int> &v) { for (int &x : v) ++x; return v; }
// <<<
// >>> rational

struct rat {
    using Int = int64_t;
    using UInt = uint64_t;
    Int a, b;
    rat() : a(0), b(1) {}
    rat(Int a, Int b = 1) {
        assert(b != 0);
        if (b < 0) a = -a, b = -b;
        auto g = gcd((UInt)abs(a), (UInt)b);
        this->a = a/g;
        this->b = b/g;
    }
    bool operator==(rat const& rhs) const {
        return a == rhs.a and b == rhs.b;
    }
    bool operator!=(rat const& rhs) const {
        return !(*this == rhs);
    }
    bool operator<(rat const& rhs) const {
        auto tmp = rhs;
        tmp -= *this;
        return tmp.a > 0;
    }
    bool operator>(rat const& rhs) const { return rhs < *this; }
    bool operator<=(rat const& rhs) const { return !(*this > rhs); }
    bool operator>=(rat const& rhs) const { return !(*this < rhs); }
    rat operator+() const { return *this; }
    rat operator-() const { return {-a, b}; }
    rat& operator+=(rat const& rhs) { return (*this) = (*this) + rhs; }
    rat& operator-=(rat const& rhs) { return (*this) = (*this) - rhs; }
    rat& operator*=(rat const& rhs) { return (*this) = (*this) * rhs; }
    rat& operator/=(rat const& rhs) { return (*this) = (*this) / rhs; }
    rat operator+(rat const& rhs) const {
        return { a*rhs.b + b*rhs.a, b*rhs.b };
    }
    rat operator-(rat const& rhs) const {
        return { a*rhs.b - b*rhs.a, b*rhs.b };
    }
    rat operator*(rat const& rhs) const {
        rat x(a, rhs.b), y(rhs.a, b);
        return { x.a*y.a, x.b*y.b };
    }
    rat inv() const { assert(a != 0); return { b, a }; };
    rat operator/(rat const& rhs) const {
        return (*this)*rhs.inv();
    }
#ifdef LOCAL
    friend string to_s(rat const& x) {
        return to_s(x.a) + "/" + to_s(x.b);
    }
#endif
};

// <<<

int32_t main() {
    int n, k; cin >> n >> k;
    vector<int> a(n); cin >> a;

    if (k == 1) quit(0);
    if (k == n) quit("1/1");

    rat p, q;
    rep (i, k-1) p += rat(1)/(a[i]*a[i+1]);
    rep (i, n-1) q += rat(1)/(a[i]*a[i+1]);
    rat ans = p/q;

    if (ans == 0) {
        cout << "0" << '\n';
    } else {
        cout << ans.a << "/" << ans.b << '\n';
    }

}