#include <iostream>
#include <fstream>
#include <iomanip>
#include <cmath>

using namespace std;

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

int n;

double s;

pair <double,double> punct[1000005];

int main() {
    int i;
    f>>n;
    for(i = 1 ;i <= n; ++i) {
        f>>punct[i].first>>punct[i].second;
    }
    for(i = 1; i < n; ++i) {
        s += punct[i].first*punct[i+1].second - punct[i].second*punct[i+1].first;
    }
    s += punct[n].first*punct[1].second - punct[n].second*punct[1].first;
    s = s/2;
    g<<fixed<<setprecision(5)<<s;
    return 0;
}