#include <cstdio>

#define normalize 2000000

using namespace std;

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

    int N;
    double xs,ys,xc,yc,xa,ya,rez = 0;

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

    printf("%.6lf\n",rez);
    return 0;
}