#include <bits/stdc++.h>
#include <boost/multiprecision/cpp_int.hpp>
using namespace std;
boost::multiprecision::cpp_int gcd(boost::multiprecision::cpp_int a, boost::multiprecision::cpp_int b){
  if (b == 0){
    return a;
  } else {
    return gcd(b, a % b);
  }
}
struct fraction{
  boost::multiprecision::cpp_int num, den;
  fraction(){
  }
  fraction(boost::multiprecision::cpp_int a, boost::multiprecision::cpp_int b){
    boost::multiprecision::cpp_int g = gcd(a, b);
    num = a / g;
    den = b / g;
  }
  fraction operator +(fraction x){
    return fraction(num * x.den + x.num * den, den * x.den);
  }
  fraction operator -(fraction x){
    return fraction(num * x.den - x.num * den, den * x.den);
  }
  fraction operator *(fraction x){
    return fraction(num * x.num, den * x.den);
  }
  fraction operator /(fraction x){
    return fraction(num * x.den, den * x.num);
  }
};
int main(){
  int N, K;
  cin >> N >> K;
  vector<int> A(N);
  for (int i = 0; i < N; i++){
    cin >> A[i];
  }
  K--;
  if (K == 0){
    cout << "0" << endl;
  } else if (K == N - 1){
    cout << "1" << endl;
  } else {
    assert(false);
    vector<fraction> dp(N);
    dp[0] = fraction(0, 1);
    dp[1] = fraction(1, 1);
    for (int i = 2; i < N; i++){
      dp[i] = fraction(A[i - 2] + A[i], A[i]) * dp[i - 1] - fraction(A[i - 2], A[i]) * dp[i - 2];
    }
    fraction x = fraction(1, 1) / dp[N - 1];
    fraction ans = x * dp[K];
    cout << ans.num << "/" << ans.den << endl;
  }
}