#include <cstdio>
#include <algorithm>
#include <cmath>

using namespace std;
FILE *f,*g;

struct bla
{
    double x,y;
}v[100002];

bool how(bla A, bla B)
{
    if(A.x!=B.x)
        return (A.x<B.x);
    return (A.y<B.y);
}
int main()
{
    f=fopen("aria.in","r");
    g=fopen("aria.out","w");
    int n;
    double arie=0;
    fscanf(f,"%d",&n);
///punctele sunt deja date in ordine trigonometrica
    for(int i=1;i<=n;++i)
        fscanf(f,"%lf %lf",&v[i].x,&v[i].y);
///pt ca sunt in ordine trigonometrica semnul ariei este acleasi la toate triunghurile
    v[n+1]=v[1];
    for(int i=2;i<=n+1;++i)
        arie=arie+v[i-1].x*v[i].y-v[i].x*v[i-1].y;
    if(arie<0)
        arie=(-arie);
    fprintf(g,"%0.1lf",arie/2);
    fclose(f);
    fclose(g);
    return 0;
}