#include <stdio.h>
#include <cmath>

using namespace std;

struct punct
{
    double x, y;
}a[100010];

int main()
{
    int n;
    double arie=0;
    FILE *f=fopen("aria.in", "r"), *g=fopen("aria.out", "w");
    fscanf(f, "%d", &n);
    for (int i=1; i<=n; i++)
        fscanf(f, "%lf%lf", &a[i].x, &a[i].y);
    for (int i=1; i<n; i++)
        arie=arie+(a[i].x*a[i+1].y-a[i].y*a[i+1].x);
    arie=arie+(a[n].x*a[1].y-a[n].y*a[1].x);
    arie=abs(arie/2);
    fprintf(g, "%.5lf", arie);
    return 0;
}