#include <stdio.h>
using namespace std;
struct POINT
{
    int x, y;
};

POINT v[100005];
double sum;
int main()
{
    int n, i;
    FILE *fin, *fout;
    fin=fopen("aria.in", "r");
    fout=fopen("aria.out", "w");
    fscanf(fin, "%d", &n);
    for(i=1; i<=n; i++)
        fscanf(fin, "%d%d", &v[i].x, &v[i].y);
    v[n+1]=v[1];
    for(i=1; i<=n; i++)
        sum=sum+(v[i].x*v[i+1].y - v[i].y*v[i+1].x);
    sum=sum/2.0;
    fprintf(fout, "%.5f", sum);
    return 0;
}