#include <cstdio>
#include <cmath>

#define NMAX 100001
using namespace std;
FILE* fin = freopen("aria.in","r",stdin);
FILE* fout = freopen("aria.out","w",stdout);
struct punct
{
    double x,y;
}puncte[NMAX];
int n;
double arie;
int main() {
    scanf("%d", &n);
    for (int i = 0; i < n; i++)
        scanf("%lf %lf",&puncte[i].x,&puncte[i].y);
    for (int i = 0; i < n; i++)
    {
        int j = (i+1) % n;
        arie += (puncte[i].x * puncte[j].y - puncte[j].x * puncte[i].y);
    }
    arie = abs(arie);
    arie *= 0.5;
    printf("%.5lf\n",arie);
    return 0;
}