#include <iostream>
#include <cstdio>
using namespace std;
 
#define NMAX 100001
 
int n;
 
struct point{
    double x,y;
}p[NMAX];
 
double area(point a, point b)
{
    return (a.y+b.y)*(b.x-a.x);
}
 
void read()
{
    scanf("%d ",&n);
    for(int i=0;i<n;i++)
    {
        scanf("%lf %lf ",&p[i].x,&p[i].y);
    }
}
 
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
 
    read();
 
    double total_area = 0;
 
 
    for(int i=0;i<n-1;i++)
        total_area += area(p[i],p[i+1]);
 
    total_area += area(p[n-1],p[0]);
 
    total_area/=2;
 
    printf("%lf",total_area >=0 ? total_area : -total_area);
 
    return 0;
}