#include <fstream>
#include<cmath>
using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");



int main()
{
    double x[100010], y[100010], s = 0;
    int n;
    fin>>n;

    for(int i = 0; i<n; ++i){
        fin>>x[i]>>y[i];
    }
    for(int i = 0; i<n-1; ++i){
        s+=(x[i]*y[i+1] - x[i+1]*y[i]);
    }
    s+=(x[n-1]*y[0]-x[0]*y[n-1]);

    s/=2;
    fout<<s;



    return 0;
}