#include <iostream>
#include <vector>

using namespace std;

int main()
{
    int c, t;
    float V = 0, S = 0, k = 0;
    cin >> c >> t;

    vector <float>a(c*2);
    vector <float>arrV(c);
    vector <float>arrS(c);

    //заполняем массив
    for (int i = 0; i < c*2; i++) {
        cin >> a[i];
    }
    //заполняем массивы S и V
    for (int i = 0; i < a.size(); i+=2){
        arrS.push_back(a[i]);
    }
    for (int i = 1; i < a.size(); i+=2){
        arrV.push_back(a[i]);
    }

    //вычисляем
    for (int i = 0; i < arrS.size(); i++){
        S = S + arrS[i];
        V = V + arrV[i];
    }

    S = S/t;
    V = V/c;
    k = S - V;

    cout << k;

    return 0;
}