//#define _GLIBCXX_DEBUG

#include<bits/stdc++.h>
using namespace std;

#define endl '\n'
#define lfs cout<<fixed<<setprecision(10)
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define spa << " " <<
#define fi first
#define se second
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define EB emplace_back
#define rep(i,n,m) for(ll i = (n); i < (ll)(m); i++)
#define rrep(i,n,m) for(ll i = (ll)(m) - 1; i >= (ll)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD1 = 1e9+7;
const ll MOD9 = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template<typename T> using PQ = priority_queue<T>;
template<typename T> using QP = priority_queue<T,vector<T>,greater<T>>;
template<typename T1, typename T2>bool chmin(T1 &a,T2 b){if(a>b){a=b;return true;}else return false;}
template<typename T1, typename T2>bool chmax(T1 &a,T2 b){if(a<b){a=b;return true;}else return false;}
ll median(ll a,ll b, ll c){return a+b+c-max({a,b,c})-min({a,b,c});}
void ans1(bool x){if(x) cout<<"Yes"<<endl;else cout<<"No"<<endl;}
void ans2(bool x){if(x) cout<<"YES"<<endl;else cout<<"NO"<<endl;}
void ans3(bool x){if(x) cout<<"Yay!"<<endl;else cout<<":("<<endl;}
template<typename T1,typename T2>void ans(bool x,T1 y,T2 z){if(x)cout<<y<<endl;else cout<<z<<endl;}  
template<typename T1,typename T2,typename T3>void anss(T1 x,T2 y,T3 z){ans(x!=y,x,z);};  
template<typename T>void debug(const T &v,ll h,ll w,string sv=" "){for(ll i=0;i<h;i++){cout<<v[i][0];for(ll j=1;j<w;j++)cout<<sv<<v[i][j];cout<<endl;}};
template<typename T>void debug(const T &v,ll n,string sv=" "){if(n!=0)cout<<v[0];for(ll i=1;i<n;i++)cout<<sv<<v[i];cout<<endl;};
template<typename T>void debug(const vector<T>&v){debug(v,v.size());}
template<typename T>void debug(const vector<vector<T>>&v){for(auto &vv:v)debug(vv,vv.size());}
template<typename T>void debug(stack<T> st){while(!st.empty()){cout<<st.top()<<" ";st.pop();}cout<<endl;}
template<typename T>void debug(queue<T> st){while(!st.empty()){cout<<st.front()<<" ";st.pop();}cout<<endl;}
template<typename T>void debug(deque<T> st){while(!st.empty()){cout<<st.front()<<" ";st.pop_front();}cout<<endl;}
template<typename T>void debug(PQ<T> st){while(!st.empty()){cout<<st.top()<<" ";st.pop();}cout<<endl;}
template<typename T>void debug(QP<T> st){while(!st.empty()){cout<<st.top()<<" ";st.pop();}cout<<endl;}
template<typename T>void debug(const set<T>&v){for(auto z:v)cout<<z<<" ";cout<<endl;}
template<typename T>void debug(const multiset<T>&v){for(auto z:v)cout<<z<<" ";cout<<endl;}
template<typename T,typename V>void debug(const map<T,V>&v){for(auto z:v)cout<<"["<<z.first<<"]="<<z.second<<endl;}
template<typename T>vector<vector<T>>vec(ll x, ll y, T w){vector<vector<T>>v(x,vector<T>(y,w));return v;}
ll gcd(ll x,ll y){ll r;while(y!=0&&(r=x%y)!=0){x=y;y=r;}return y==0?x:y;}
vector<ll>dx={1,-1,0,0,1,1,-1,-1};vector<ll>dy={0,0,1,-1,1,-1,1,-1};
template<typename T>vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>auto make_v(size_t a,Ts... ts){return vector<decltype(make_v(ts...))>(a,make_v(ts...));}
template<typename T1, typename T2>ostream &operator<<(ostream &os, const pair<T1, T2>&p){return os << p.first << " " << p.second;}
template<typename T>ostream &operator<<(ostream &os, const vector<T> &v){for(auto &z:v)os << z << " ";cout<<"|"; return os;}
template<typename T>void rearrange(vector<int>&ord, vector<T>&v){
  auto tmp = v;
  for(int i=0;i<tmp.size();i++)v[i] = tmp[ord[i]];
}
template<typename Head, typename... Tail>void rearrange(vector<int>&ord,Head&& head, Tail&&... tail){
  rearrange(ord, head);
  rearrange(ord, tail...);
}
template<typename T> vector<int> ascend(const vector<T>&v){
  vector<int>ord(v.size());iota(ord.begin(),ord.end(),0);
  sort(ord.begin(),ord.end(),[&](int i,int j){return v[i]<v[j];});
  return ord;
}
template<typename T> vector<int> descend(const vector<T>&v){
  vector<int>ord(v.size());iota(ord.begin(),ord.end(),0);
  sort(ord.begin(),ord.end(),[&](int i,int j){return v[i]>v[j];});
  return ord;
}
ll FLOOR(ll n,ll div){return n>=0?n/div:(n-div+1)/div;}
ll CEIL(ll n,ll div){return n>=0?(n+div-1)/div:n/div;}
template<typename T>T min(const vector<T>&v){return *min_element(v.begin(),v.end());}
template<typename T>T max(const vector<T>&v){return *max_element(v.begin(),v.end());}
template<typename T>T acc(const vector<T>&v){return accumulate(v.begin(),v.end(),T(0));};
template<typename T>T reverse(const T &v){return T(v.rbegin(),v.rend());};
//mt19937 mt(chrono::steady_clock::now().time_since_epoch().count());
int popcount(ll x){return __builtin_popcountll(x);};
int poplow(ll x){return __builtin_ctzll(x);};
int pophigh(ll x){return 63 - __builtin_clzll(x);};
template<typename T>T poll(queue<T> &q){auto ret=q.front();q.pop();return ret;};
template<typename T>T poll(priority_queue<T> &q){auto ret=q.top();q.pop();return ret;};
template<typename T>T poll(QP<T> &q){auto ret=q.top();q.pop();return ret;};
template<typename T>T poll(stack<T> &s){auto ret=s.top();s.pop();return ret;};
template< typename T = int >
struct edge {
  int to;
  T cost;
  int id;
  edge():id(-1){};
  edge(int to, T cost = 1, int id = -1):to(to), cost(cost), id(id){}
  operator int() const { return to; }
};

template<typename T>
using Graph = vector<vector<edge<T>>>;
template<typename T>
Graph<T> readGraph(int n,int m,int indexed=1,bool directed=false,bool weighted=false){
  Graph<T> ret(n);
  for(int es = 0; es < m; es++){
    int u,v,w=1;
    cin>>u>>v;u-=indexed,v-=indexed;
    if(weighted)cin>>w;
    ret[u].emplace_back(v,w,es);
    if(!directed)ret[v].emplace_back(u,w,es);
  }
  return ret;
}
template<typename T>
Graph<T> readParent(int n,int indexed=1,bool directed=true){
  Graph<T>ret(n);
  for(int i=1;i<n;i++){
    int p;cin>>p;
    p-=indexed;
    ret[p].emplace_back(i);
    if(!directed)ret[i].emplace_back(p);
  }
  return ret;
}
struct RationalNumber{
  long long a, b;
  RationalNumber(){
    a = 0,b = 1;
  }
  RationalNumber(long long p, long long q):a(p), b(q){// a / b
    reduction();
  };
  RationalNumber(long long p):a(p), b(1){// a / b
  };
  void reduction(){
    long long g = __gcd(a, b);
    a /= g, b /= g;
    if(b < 0)a *= -1,b *= -1;
  }
  RationalNumber &operator+=(const RationalNumber &x){
    long long q = b / __gcd(b, x.b) * x.b;
    a = a * (q / b) + x.a * (q / x.b);
    b = q;
    reduction();
    return *this;
  }
  RationalNumber &operator-=(const RationalNumber &x){
    long long q = b / __gcd(b, x.b) * x.b;
    a = a * (q / b) - x.a * (q / x.b);
    b = q;
    reduction();
    return *this;
  }
  RationalNumber &operator*=(const RationalNumber &x){
    long long q1 = __gcd(a, x.b), q2 = __gcd(b, x.a); 
    a = (a / q1) * (x.a / q2), b = (b / q2) * (x.b / q1); 
    return *this;
  }
  RationalNumber &operator/=(const RationalNumber &x){
    long long q1 = __gcd(a, x.a), q2 = __gcd(b, x.b); 
    a = (a / q1) * (x.b / q2), b = (b / q2) * (x.a / q1); 
    return *this;
  }
  RationalNumber operator+(const RationalNumber &x)const{
    return RationalNumber(*this) += x;
  }
  RationalNumber operator-(const RationalNumber &x)const{
    return RationalNumber(*this) -= x;
  }
  RationalNumber operator*(const RationalNumber &x)const{
    return RationalNumber(*this) *= x;
  }
  RationalNumber operator/(const RationalNumber &x)const{
    return RationalNumber(*this) /= x;
  }
  RationalNumber operator-()const{
    return RationalNumber(-a, b);
  }
  bool operator==(const RationalNumber &x)const{
    return a * x.b == b * x.a;
  };
  bool operator!=(const RationalNumber &x)const{
    return !(*this == x);
  };
  bool operator<(const RationalNumber &x)const{
    return a * x.b < b * x.a;
  }
  bool operator>(const RationalNumber &x)const{
    return a * x.b > b * x.a;
  }
  bool operator<=(const RationalNumber &x)const{
    return *this < x || *this == x;
  }
  bool operator>=(const RationalNumber &x)const{
    return *this > x || *this == x;
  }
  friend ostream &operator<<(ostream &os, RationalNumber x){
    return os << x.a << "/" << x.b;
  }
};
using rat=RationalNumber;
int main(){
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  ll res=0,buf=0;
  bool judge = true;
  ll n,k;cin>>n>>k;k--;
  vector<ll>a(n);
  rep(i,0,n)cin>>a[i];
  vector<rat>b(n),c(n);
  b[0]=1,c[0]=0;
  rep(i,1,n-1){
    rat p(a[i-1],a[i+1]+a[i-1]);
    rat q(a[i+1],a[i+1]+a[i-1]);
    b[i]=p*b[i-1]/(rat(1)-p*c[i-1]);
    c[i]=q/(rat(1)-p*c[i-1]);
  }
  vector<rat>d(n);
  d[n-1]=0;
  rrep(i,0,n-1){
    d[i]=b[i]+c[i]*d[i+1];
  }
  cout<<rat(1)-d[k]<<endl;
  return 0;
}