#include <fstream>
#include <stdlib.h>
#include <iomanip>

using namespace std;

ifstream cin("aria.in");
ofstream cout("aria.out");

int n; double p[100005][2];
double aria;

int main()
{
    cin>>n;
    for (int i=0;i<n;i++) cin>>p[i][0]>>p[i][1];

    for(int i = 1; i+1<n; i++){
        int x1 = p[i][0] - p[0][0];
        int y1 = p[i][1] - p[0][1];
        int x2 = p[i+1][0] - p[0][0];
        int y2 = p[i+1][1] - p[0][1];
        int cross = x1*y2 - x2*y1;
        aria += cross;
    }

    cout<<fixed;
    cout<<setprecision(5)<<abs(aria)/2<<'\n';
    cin.close();cout.close();
    return 0;
}