#pragma GCC optimize ("O2")
#pragma GCC target ("avx")
#include<bits/stdc++.h>
//#include<atcoder/all>
//using namespace atcoder;

using namespace std;
typedef long long ll;
#define rep(i, n) for(int i = 0; i < (n); i++)
#define rep1(i, n) for(int i = 1; i <= (n); i++)
#define co(x) cout << (x) << "\n"
#define cosp(x) cout << (x) << " "
#define ce(x) cerr << (x) << "\n"
#define cesp(x) cerr << (x) << " "
#define pb push_back
#define mp make_pair
#define chmin(x, y) x = min(x, y)
#define chmax(x, y) x = max(x, y)
#define Would
#define you
#define please

ll gcd(ll A, ll B) {
	if (B == 0) return A;
	return gcd(B, A % B);
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);


	ll are = 1;
	rep1(i, 10) rep1(j, 10) {
		are = are / gcd(are, i * j) * i * j;
	}
	ce(are);

	int N, K;
	cin >> N >> K;

	if (N == 1) {
		co("1/1");
		return 0;
	}
	if (K == 1) {
		co(0);
		return 0;
	}

	int A[100000];
	rep(i, N) cin >> A[i];

	ll bunsi = 0;
	rep(i, K - 1) {
		bunsi += are / A[i] / A[i + 1];
	}
	ll bunbo = 0;
	rep(i, N - 1) {
		bunbo += are / A[i] / A[i + 1];
	}
	ll g = gcd(bunsi, bunbo);

	cout << bunsi / g << '/' << bunbo / g << endl;

	Would you please return 0;
}