#include <fstream>
//#include <iomanip>
using namespace std;
//ifstream fin("aria.in");
//ofstream fout("aria.out");
FILE *fin = fopen("aria.in", "r");
FILE *fout = fopen("aria.out", "w");
double arie(int a, int b, int c);
int n, i;
double x[100001], y[100001], sum;
int main()
{
    //fin>>n;
    fscanf(fin, "%d", &n);

    for(i=1; i<=n; i++)
        fscanf(fin, "%lf %lf", &x[i], &y[i]);
    //    fin>>x[i]>>y[i];
    for(i=2; i<=n-1; i++)
        sum+=arie(1, i, i+1);
    if(sum<0)
        sum=sum*-1;
    //fout<<setprecision(6)<<sum;
    fprintf(fout, "%.5lf\n", sum);
    fclose(fout);
    return 0;
}
double arie(int a, int b, int c)
{
    double f = 0;
    f=x[a]*y[b]+y[a]*x[c]+x[b]*y[c]-y[b]*x[c]-y[a]*x[b]-x[a]*y[c];
    return f/2;
}