#include <cstdio>
#define NMAX 100005

using namespace std;

double x[NMAX],y[NMAX];
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",&x[i],&y[i]);
    }
    x[0]=x[n];y[0]=y[n];
    double s = 0;
    for(int i = 1; i <= n; ++i)
    {
        s += ((x[i-1] * y[i]) - (x[i] * y[i-1]));
    }
    s = s / 2;
    printf("%.5lf",s);
    return 0;
}