#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#define nmax 100005
using namespace std;

int n;
struct pct
{
    double x,y;
} p[nmax];

double sol;

int main()
{
    int i;
    pct prec,act,prim;
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for(i=0;i<n;i++) scanf("%lf%lf",&p[i].x,&p[i].y);
    p[n]=p[0];
    for(i=0;i<n;i++)
        sol+=( p[i].x*p[i+1].y - p[i].y*p[i+1].x );
    printf("%lf\n",fabs(sol/2.0));
    fclose(stdin);
    fclose(stdout);
    return 0;
}