#include <iostream>
#include <cstdio>
#define NMAX 100005
using namespace std;

struct punct
{
    double x,y;
}puncte[NMAX];
int N;
double rez;

void citire()
{
    scanf("%d",&N);
    for(int i=1;i<=N;i++)
        scanf("%lf %lf",&puncte[i].x,&puncte[i].y);
}

double cross_product(punct p1,punct p2)
{
    return p1.x*p2.y - p1.y*p2.x;
}
void calculare()
{
    for(int i=1;i<N;i++)
        rez+=cross_product(puncte[i],puncte[i+1]);
    rez+=cross_product(puncte[N],puncte[1]);
    rez/=2;
    printf("%.5lf\n",rez);
}

int main()
{
    freopen("aria.in","r",stdin);
    citire();
    calculare();
    return 0;
}