#include <iostream>
#include <numeric>
#include <vector>

using namespace std;
using lint = long long;

constexpr lint G = 6350400;  // LCM(1,2,...,10)^2

void solve() {
    int n, k;
    cin >> n >> k;
    --k;

    if (k == 0) {
        cout << "0\n";
        return;
    }

    vector<lint> xs(n);
    for (auto& x : xs) cin >> x;

    lint num = 0, dom = 0;
    for (int i = 1; i <= k; ++i) {
        num += G / xs[i] / xs[i - 1];
    }
    for (int i = 1; i < n; ++i) {
        dom += G / xs[i] / xs[i - 1];
    }

    {
        auto g = gcd(num, dom);
        num /= g, dom /= g;
    }

    cout << num << "/" << dom << "\n";
}

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

    solve();

    return 0;
}