#include <cstdio>

using namespace std;
const int eps = 1.e-14;
int main()
{   freopen("aria.in", "r",stdin);
    freopen("aria.out", "w",stdout);
    double x1,y1,x2,y2,a;
    int i,n;
    scanf("%d", &n);
    scanf("%lf%lf", &x1, &y1);
    for(i=2; i<=n;i++){
        scanf("%lf%lf", &x2, &y2);
        a+=(x1-x2)*(y1+y2);
        x1=x2;
        y1=y2;
    }
    a/=2;
    if(a<eps)
        a*=-1;
    printf("%.5lf", a);
    return 0;
}