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

int main()
{
    int n, k;
    cin >> n >> k;
    vector<int> a(n);
    for (int i = 0; i < n; i++)
    {
        cin >> a.at(i);
    }
    const int root = 2520 * 2520;
    vector<int> d(n - 1);
    for (int i = 0; i < n - 1; i++)
    {
        d.at(i) = root / (a.at(i) * a.at(i + 1));
    }
    int64_t p = accumulate(d.begin(), d.begin() + k - 1, int64_t()), q = accumulate(d.begin(), d.end(), int64_t());
    int64_t g = gcd(p, q);
    p /= g;
    q /= g;
    if (p == 0)
    {
        cout << 0 << endl;
    }
    else
    {
        cout << p << '/' << q << endl;
    }
    return 0;
}