#include <bits/stdc++.h>
using namespace std;
using ld = long double;
ld read() {
    string s;
    cin >> s;
    return stold(s);
}
ld pt[2505][2], mul[2505];
bool num[2505], den[2505];
int main() {
    ios::sync_with_stdio(0); cin.tie(0);
    int n,k;
    cin >> n >> k;
    auto prob = [&](int i) {
        for (int j = 0; j < k-2; j++) {
            num[n-i-2-j] = true;
            den[n-j] = true;
        }
        vector<int> remn, remd;
        for (int j = 0; j < 2505; j++) {
            if (num[j] == den[j]) continue;
            if (num[j]) remn.push_back(j);
            if (den[j]) remd.push_back(j);
        }
        ld ret = k*(k-1)/2;
        assert(remn.size() == remd.size());
        while (remn.size()) {
            ret *= remn.back();
            ret /= remd.back();
            remn.pop_back();
            remd.pop_back();
        }
        for (int j = 0; j < k-2; j++) {
            num[n-i-2-j] = false;
            den[n-j] = false;
        }
        ret *= 2;
        ret /= ((n-k+2)*(n-k+1));
        return ret;
    };
    for (int i = 0; i < n; i++) {
        pt[i][0] = read();
        pt[i][1] = read();
    }
    vector<int> ind(n);
    iota(ind.begin(),ind.end(),0);
    reverse(ind.begin(),ind.end());
    ld tot = 0;
    for (int i = 0; i <= n-k; i++) {
        ld pos = 0, neg = 0;
        for (int j = 0; j < n; j++) {
            int l = (j+i+1)%n;
            pos += pt[ind[j]][0]*pt[ind[l]][1];
            neg += pt[ind[j]][1]*pt[ind[l]][0];
        }
        ld ret = prob(i)*(pos-neg);
        tot += ret;
    }
    cout << setprecision(20) << tot/2 << '\n';
    return 0;
}