#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=1;i<=n;i++) cin>>p[i][0]>>p[i][1];

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


    cout<<fixed;
    cout<<setprecision(6)<<aria/2<<'\n';
    cin.close();cout.close();
    return 0;
}