#include <iostream>
#include <vector>
#include <fstream>
#include <iomanip>

using namespace std;

struct punct{
    long double x, y;
    punct(long double x = 0, long double y = 0){
        this->x = x;
        this->y = y;
    }
};

int n;
vector<punct> puncte;

ifstream f("aria.in");
ofstream g("aria.out");

long double determinant(punct a, punct b){
    long double rez = (a.x) * (b.y) - (a.y) * (b.x);
    // if(rez < 0)
        // rez *= -1;
    return rez;
}

int main() {
    f>>n;
    for (int i = 0; i < n; ++i) {
        puncte.emplace_back();
        f>>puncte[i].x>>puncte[i].y;
    }
    puncte.push_back(puncte[0]);
    long double aria2 = 0;
    for (int i = 0; i < n; ++i) {
        aria2 += determinant(puncte[i], puncte[i + 1]);
    }
    aria2 /= 2;
    g<<setprecision(15)<< fixed << aria2;
    return 0;
}