#include <cstdio>

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


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


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

    return 0;
}