#include <cstdio>
#include <cmath>
using namespace std;
const int Max = 100007;
struct loc
{
    double x, y;
} v[Max];
double sum;
int n;
int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    scanf("%d", &n);
    for(int i = 1; i <= n; ++i)
    {
        scanf("%lf%lf",&v[i].x,&v[i].y);
    }
    v[n + 1].x = v[1].x;
    v[n + 1].y = v[1].y;
    for(int i = 1; i <= n; ++i)
    {
        sum+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    }
    printf("%lf",abs(sum)/2.0);
    return 0;
}