#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using uint = unsigned int;
using ull = unsigned long long;
#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 per(i,n) for(int i=int(n)-1;i>=0;i--)
#define per1(i,n) for(int i=int(n);i>0;i--)
#define all(c) c.begin(),c.end()
#define si(x) int(x.size())
#define pb emplace_back
#define fs first
#define sc second
template<class T> using V = vector<T>;
template<class T> using VV = vector<vector<T>>;
template<class T,class U> void chmax(T& x, U y){if(x<y) x=y;}
template<class T,class U> void chmin(T& x, U y){if(y<x) x=y;}
template<class T> void mkuni(V<T>& v){sort(all(v));v.erase(unique(all(v)),v.end());}
template<class T> int lwb(const V<T>& v, const T& a){return lower_bound(all(v),a) - v.begin();}
template<class S,class T> ostream& operator<<(ostream& o,const pair<S,T> &p){
	return o<<"("<<p.fs<<","<<p.sc<<")";
}
template<class T> ostream& operator<<(ostream& o,const vector<T> &vc){
	o<<"{";
	for(const T& v:vc) o<<v<<",";
	o<<"}";
	return o;
}
constexpr ll TEN(int n) { return (n == 0) ? 1 : 10 * TEN(n-1); }

#ifdef LOCAL
#define show(x) cerr << "LINE" << __LINE__ << " : " << #x << " = " << (x) << endl
void dmpr(ostream& os){os<<endl;}
template<class T,class... Args>
void dmpr(ostream&os,const T&t,const Args&... args){
	os<<t<<" ~ ";
	dmpr(os,args...);
}
#define shows(...) cerr << "LINE" << __LINE__ << " : ";dmpr(cerr,##__VA_ARGS__)
#define dump(x) cerr << "LINE" << __LINE__ << " : " << #x << " = {";  \
	for(auto v: x) cerr << v << ","; cerr << "}" << endl;
#else
#define show(x) void(0)
#define dump(x) void(0)
#define shows(...) void(0)
#endif

int bsf(ll x) { return __builtin_ctzll(x); } 
ll gcd(ll a, ll b){
	a = abs(a), b = abs(b);
	if(a==0) return b;
	if(b==0) return a;
	int shift = bsf(a|b);
	a >>= bsf(a);
	do{
		b >>= bsf(b);
		if(a>b) swap(a,b);
		b -= a;
	}while(b);
	return a<<shift;
}

struct Frac{
	ll x,y;		// x/y
	Frac(ll x_ = 0):x(x_),y(1){}
	Frac(ll x_,ll y_){
		ll g = gcd(x_,y_);
		if(y_ < 0) g = -g;
		x = x_ / g;
		y = y_ / g;
	}

	Frac operator-() const { return {-x,y}; }
	Frac operator+(const Frac& r) const { return {x * r.y + y * r.x, y * r.y}; }
	Frac operator-(const Frac& r) const { return *this + (-r); }
	Frac operator*(const Frac& r) const { return {x * r.x, y * r.y}; }
	Frac operator/(const Frac& r) const { return {x * r.y, y * r.x}; }
	Frac& operator+=(const Frac& r) { return *this = *this + r; }
	Frac& operator-=(const Frac& r) { return *this = *this - r; }
	Frac& operator*=(const Frac& r) { return *this = *this * r; }
	Frac& operator/=(const Frac& r) { return *this = *this / r; }
	bool operator<(const Frac& r) const { return x * r.y < y * r.x; }
	bool operator>(const Frac& r) const { return r < *this; }
	bool operator<=(const Frac& r) const { return !(r < *this); }
	bool operator>=(const Frac& r) const { return !(*this < r); }
	bool operator==(const Frac& r) const { return x == r.x && y == r.y; }
	bool operator!=(const Frac& r) const { return !(*this == r); }
	friend ostream& operator<<(ostream &o,const Frac& x){
		return o << x.x << "/" << x.y;
	}
};

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);		//DON'T USE scanf/printf/puts !!
	cout << fixed << setprecision(20);

	int N,K; cin >> N >> K; K--;
	V<int> A(N); rep(i,N) cin >> A[i];
	V<Frac> f(N);
	f[N-1] = 1;
	if(N > 2){
		V<Frac> a(N),b(N);
		// f[1] = x
		// f[i] = a[i]x + b[i]
		a[1] = 1;
		for(int i=2;i<N;i++){
			a[i] = Frac(A[i-2]+A[i],A[i]) * a[i-1] - Frac(A[i-2],A[i]) * a[i-2];
			b[i] = Frac(A[i-2]+A[i],A[i]) * b[i-1] - Frac(A[i-2],A[i]) * b[i-2];
		}
		Frac x = (Frac(1)-b[N-1])/a[N-1];
		Frac res = a[K]*x + b[K];
		if(res == 0) cout << 0 << endl;
		else cout << res << endl;
	}else{
		if(K == N-1) cout << "1/1" << endl;
		else cout << 0 << endl;
	}
}