#include<bits/stdc++.h>
using namespace std;
#include<atcoder/all>
using namespace atcoder;
// input and output of modint
istream &operator>>(istream &is, modint998244353 &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint998244353 &a) { return os << a.val(); }
istream &operator>>(istream &is, modint1000000007 &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint1000000007 &a) { return os << a.val(); }
template<int m> istream &operator>>(istream &is, static_modint<m> &a) { long long v; is >> v; a = v; return is; }
template<int m> ostream &operator<<(ostream &os, const static_modint<m> &a) { return os << a.val(); }
template<int m> istream &operator>>(istream &is, dynamic_modint<m> &a) { long long v; is >> v; a = v; return is; }
template<int m> ostream &operator<<(ostream &os, const dynamic_modint<m> &a) { return os << a.val(); }
#define rep_(i, a_, b_, a, b, ...) for (int i = (a), lim##i = (b); i < lim##i; ++i)
#define rep(i, ...) rep_(i, __VA_ARGS__, __VA_ARGS__, 0, __VA_ARGS__)
#define drep_(i, a_, b_, a, b, ...) for (int i = (a)-1, lim##i = (b); i >= lim##i; --i)
#define drep(i, ...) drep_(i, __VA_ARGS__, __VA_ARGS__, __VA_ARGS__, 0)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#ifdef LOCAL
void debug_out() { cerr << endl; }
template <class Head, class... Tail> void debug_out(Head H, Tail... T) { cerr << ' ' << H; debug_out(T...); }
#define debug(...) cerr << 'L' << __LINE__ << " [" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#define dump(x) cerr << 'L' << __LINE__ << " " << #x << " = " << (x) << endl
#else
#define debug(...) (void(0))
#define dump(x) (void(0))
#endif
template<class T> using V = vector<T>;
using ll = long long;
using ld = long double;
using Vi = V<int>;  using VVi = V<Vi>;
using Vl = V<ll>;   using VVl = V<Vl>;
using Vd = V<ld>;   using VVd = V<Vd>;
using Vb = V<bool>; using VVb = V<Vb>;
template<class T> using priority_queue_rev = priority_queue<T, vector<T>, greater<T>>;
template<class T> vector<T> make_vec(size_t n, T a) { return vector<T>(n, a); }
template<class... Ts> auto make_vec(size_t n, Ts... ts) { return vector<decltype(make_vec(ts...))>(n, make_vec(ts...)); }
template<class T> inline int sz(const T &x) { return x.size(); }
template<class T> inline bool chmin(T &a, const T b) { if (a > b) { a = b; return true; } return false; }
template<class T> inline bool chmax(T &a, const T b) { if (a < b) { a = b; return true; } return false; }
template<class T1, class T2> istream &operator>>(istream &is, pair<T1, T2> &p) { is >> p.first >> p.second; return is; }
template<class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p) { os << '(' << p.first << ", " << p.second << ')'; return os; }
template<class T, size_t n> istream &operator>>(istream &is, array<T, n> &v) { for (auto &e : v) is >> e; return is; }
template<class T, size_t n> ostream &operator<<(ostream &os, const  array<T, n> &v) { for (auto &e : v) os << e << ' '; return os; }
template<class T> istream &operator>>(istream &is, vector<T> &v) { for (auto &e : v) is >> e; return is; }
template<class T> ostream &operator<<(ostream &os, const vector<T> &v) { for (auto &e : v) os << e << ' '; return os; }
template<class T> inline void deduplicate(vector<T> &a) { sort(all(a)); a.erase(unique(all(a)), a.end()); }
template<class T> inline int count_between(const vector<T> &a, T l, T r) { return lower_bound(all(a), r) - lower_bound(all(a), l); } // [l, r)
inline ll ceil_div(const ll x, const ll y) { return (x+y-1) / y; } // ceil(x/y)
inline int floor_log2(const ll x) { assert(x > 0); return 63-__builtin_clzll(x); } // floor(log2(x))
inline int ceil_log2(const ll x) { assert(x > 0); return (x == 1) ? 0 : 64-__builtin_clzll(x-1); } // ceil(log2(x))
inline int popcount(const ll x) { return __builtin_popcountll(x); }
inline void fail() { cout << -1 << '\n'; exit(0); }
struct fast_ios { fast_ios(){ cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
// const int INF  = (1<<30) - 1;
// const ll INFll = (1ll<<60) - 1;
// const ld EPS   = 1e-10;
// const ld PI    = acos(-1.0);
// using mint = modint998244353;
// using mint = modint1000000007;
// using mint = modint;
// using Vm = V<mint>; using VVm = V<Vm>;



template<class T>
struct rational {
  T p, q;
  rational(T _p = 0, T _q = 1, bool red = true) : p(_p), q(_q) {
    if (q != 1 && red) reduce(p, q);
  }
  void reduce(T &a, T &b) {
    T g = gcd(a, b);
    a /= g, b /= g;
    if (b < 0) a = -a, b = -b;
    else if (a < 0 && b == 0) a = -a;
  }
  bool operator<(const rational &r) const { return p * r.q < r.p * q; }
  rational& operator+=(const rational &r) { p = p * r.q + r.p * q, q *= r.q; reduce(p, q); return *this; }
  rational& operator-=(const rational &r) { p = p * r.q - r.p * q, q *= r.q; reduce(p, q); return *this; }
  rational& operator*=(const rational &r) { p *= r.p, q *= r.q; reduce(p, q); return *this; }
  rational& operator/=(const rational &r) { p *= r.q, q *= r.p; reduce(p, q); return *this; }
  rational operator+(const rational &r) const { return rational(*this) += r; }
  rational operator-(const rational &r) const { return rational(*this) -= r; }
  rational operator*(const rational &r) const { return rational(*this) *= r; }
  rational operator/(const rational &r) const { return rational(*this) /= r; }
};
template<class T> ostream &operator<<(ostream &os, const rational<T> &a) { return os << a.p << '/' << a.q; }

using rat = rational<ll>;


void solve() {
  int n, k; cin >> n >> k;
  Vl a(n); cin >> a;

  V<rat> x(n);
  x[1] = 1;

  rep(i, 2, n) x[i] = x[i-1] * rat{a[i-2]+a[i], a[i]} - x[i-2] * rat{a[i-2], a[i]};
  auto ans = x[k-1] / x[n-1];

  if (ans.p == 0) cout << 0 << '\n';
  else cout << ans << '\n';
}


int main() {
  solve();
}