#include <iostream>
#include <vector>

using namespace std;

int silnia(int n)
{
    if(n<=1)
        return 1;
    else
        return silnia(n-1)*n;
}


int main(int argc, char** argv) {
    double x0=2.5;
    double x[] = {1,2,3,4};
    double y[] = {3,7,8,15};
    double h=x[1]-x[0];
    vector<double> wsp;
    int n=4;
    wsp.push_back(y[0]);
    for(int i=0;i<(n-1);i++)
    {
        for(int j=0;j<(n-1-i);j++)
           y[j]=y[j+1]-y[j];
        wsp.push_back(y[0]);
    }

    double l;
    double s=0;
    for(int i=0;i<wsp.size();i++)
    {
        l=1;
        for(int j=0;j<i;j++)
        {
            l*=(x0-x[j]);
        }
        s+=wsp[i]*l/(silnia(i)*h);
        h*=h;
    }

    cout << "f(" << x0 << ") = " << s << endl;

    return 0;
}