#include <iostream>
#include <cstdio>

using namespace std;

int n;
double s;

struct punct
{
    double x,y;
}a[100001];

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

    scanf("%d\n",&n);
    for(int i=0;i<n;i++)
        scanf("%lf %lf\n",&a[i].x, &a[i].y);
    for(int i=0;i<n-1;i++)
        s=s+(a[i].x*a[i+1].y-a[i+1].x*a[i].y)/2;
    s=s+(a[n-1].x*a[0].y-a[0].x*a[n-1].y)/2;
    printf("%lf\n",s);
    return 0;
}