#include <cstdio>


using namespace std;
long double xs,ys,xc,yc,xa,ya;
long double rez = 0;
int N;


int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);

    double auX,auY;
    scanf("%d",&N);
    scanf("%lf%lf",&auX,&auY);
    xs = auX;ys = auY;
    xa = xs;
    ya = ys;
    for(int i = 2; i <= N; ++i){
        scanf("%lf%lf",&auX,&auY);
        xc = auX; yc = auY;
        rez += (ya + yc)*(xa - xc);
        xa = xc;
        ya = yc;
    }
    rez += (ya + ys)*(xa - xs);
    rez /= 2;
    auX = rez;
    printf("%.8lf",auX);

    return 0;
}