#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep2(i, x, n) for(int i = x; i <= n; i++)
#define rep3(i, x, n) for(int i = x; i >= n; i--)
#define each(e, v) for(auto &e: v)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) (int)x.size()
using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
const int MOD = 1000000007;
//const int MOD = 998244353;
const int inf = (1<<30)-1;
const ll INF = (1LL<<60)-1;
template<typename T> bool chmax(T &x, const T &y) {return (x < y)? (x = y, true) : false;};
template<typename T> bool chmin(T &x, const T &y) {return (x > y)? (x = y, true) : false;};

struct io_setup{
    io_setup(){
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout << fixed << setprecision(15);
    }
} io_setup;

pll mul(pll a, pll b){
    ll p = a.first*b.first, q = a.second*b.second;
    ll g = gcd(p, q);
    return pll(p/g, q/g);
}

pll add(pll a, pll b){
    ll p = a.first*b.second+a.second*b.first, q = a.second*b.second;
    ll g = gcd(p, q);
    return pll(p/g, q/g);
}

int main(){
    int N, K; cin >> N >> K; K--;

    vector<ll> A(N);
    rep(i, N) cin >> A[i];
    if(N == 1) {cout << "1/1\n"; return 0;}

    vector<pll> dp(N);
    dp[0] = pll(0, 1), dp[1] = pll(1, 1);
    pll p = dp[1];

    rep2(i, 2, N-1){
        pll q = mul(p, pll(A[i-2], A[i]));
        dp[i] = add(dp[i-1], q);
        p = q;
    }
    /*
    rep(i, N){
        cout << dp[i].first << ' ' << dp[i].second << '\n';
    }
    */
    pll ans = mul(dp[K], pll(dp[N-1].second, dp[N-1].first));

    if(ans.first == 0) cout << "0\n";
    else cout << ans.first << '/' << ans.second << '\n';
}