#include <bits/stdc++.h>

struct rational {
public:
	rational() = default;
	rational(long long x, long long y) : x(x), y(y) {}

	rational& operator+=(const rational& oth) {
		auto [z, w] = oth;
		auto ny = y / std::gcd(y, w) * w;
		auto nx = ny / y * x + ny / w * z;
		x = nx, y = ny;
		reduction();
		return *this;
	}
	rational& operator/=(const rational& oth) {
		auto [z, w] = oth;
		x *= w, y *= z;
		reduction();
		return *this;
	}
	rational operator/(rational oth) {
		return rational(*this) /= oth;
	}

	friend std::ostream& operator<<(std::ostream& s, rational a) {
		if(a.x == 0 and a.y == 1)
			s << '0';
		else
			s << a.x << '/' << a.y;
		return s;
	}

private:
	long long x, y;

	void reduction() {
		long long g = std::gcd(x, y);
		x /= g, y /= g;
	}
};

int n, k;
int a[100000];
int main() {
	std::cin >> n >> k;
	for(int i = 0; i < n; i++) std::cin >> a[i];

	auto f = [&](int x) {
		rational res(0, 1);
		for(int i = 0; i < x; i++)
			res += rational(a[0] * a[1], a[i] * a[i + 1]);
		return res;
	};

	std::cout << (n != 1 ? f(--k) / f(--n) : rational(1, 1)) << '\n';
}