#include <cstdio>
#include<cmath>
#define MAX 100000
using namespace std;
struct aa{
    double x, y;};
aa v[MAX+2];
int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int n, i;
    double a=0;
    scanf("%d", &n);
    for(i=1;i<=n;i++)
        scanf("%lf%lf", &v[i].x, &v[i].y);
    v[n+1]=v[1];
    for(i=1;i<=n;i++)
        a+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    printf("%lf", fabs((a/2.0)));
    return 0;
}