// >>> TEMPLATES #include 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 constexpr auto mp(A &&a, B &&b) { return make_pair(forward(a), forward(b)); } template constexpr auto mt(T&&... x) { return make_tuple(forward(x)...); } template auto constexpr inf_ = numeric_limits::max()/2-1; auto constexpr INF32 = inf_; auto constexpr INF64 = inf_; auto constexpr INF = inf_; #ifdef LOCAL #include "debug.hpp" #else #define dump(...) (void)(0) #define say(x) (void)(0) #define debug if (0) #endif template struct pque : priority_queue, Comp> { vector &data() { return this->c; } void clear() { this->c.clear(); } }; template using pque_max = pque>; template using pque_min = pque>; template ::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 ::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 = typename enable_if::value>::type> istream& operator>>(istream& is, T &a) { for (auto& x : a) is >> x; return is; } template ostream& operator<<(ostream& os, pair const& p) { return os << p.first << " " << p.second; } template istream& operator>>(istream& is, pair& p) { return is >> p.first >> p.second; } struct IOSetup { IOSetup() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } } iosetup; template struct FixPoint : private F { constexpr FixPoint(F&& f) : F(forward(f)) {} template constexpr auto operator()(T&&... x) const { return F::operator()(*this, forward(x)...); } }; struct MakeFixPoint { template constexpr auto operator|(F&& f) const { return FixPoint(forward(f)); } }; #define MFP MakeFixPoint()| #define def(name, ...) auto name = MFP [&](auto &&name, __VA_ARGS__) template struct vec_impl { using type = vector::type>; template static type make_v(size_t n, U&&... x) { return type(n, vec_impl::make_v(forward(x)...)); } }; template struct vec_impl { using type = T; static type make_v(T const& x = {}) { return x; } }; template using vec = typename vec_impl::type; template auto make_v(Args&&... args) { return vec_impl::make_v(forward(args)...); } template void quit(T const& x) { cout << x << endl; exit(0); } template constexpr bool chmin(T& x, U const& y) { if (x > y) { x = y; return true; } return false; } template constexpr bool chmax(T& x, U const& y) { if (x < y) { x = y; return true; } return false; } template constexpr auto sumof(It b, It e) { return accumulate(b, e, typename iterator_traits::value_type{}); } template int sz(T const& x) { return x.size(); } template int lbd(C const& v, T const& x) { return lower_bound(begin(v), end(v), x)-begin(v); } template 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 Int rand(Int a, Int b) { return uniform_int_distribution(a, b)(seed_); } i64 irand(i64 a, i64 b) { return rand(a, b); } // [a, b] u64 urand(u64 a, u64 b) { return rand(a, b); } // template void shuffle(It l, It r) { shuffle(l, r, seed_); } vector &operator--(vector &v) { for (int &x : v) --x; return v; } vector &operator++(vector &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 a(n); cin >> a; if (k == n) quit("1/1"); if (k == 1) quit(0); 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'; } }