#include <iostream>
#include <cstdio>
#define cout cerr
#define MAX 100001
using namespace std;

pair <double, double> pun[MAX];
int n;
double x,y,A;

double Arie(double xa,double ya, double xb,double yb)
{
    return (ya+yb)*(xa-xb)/2;
}

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

    scanf("%d",&n);

    for(int i=1;i<=n;i++)
    {
        scanf("%lf %lf",&x,&y);
        pun[i]=make_pair(x,y);
    }

    for(int i=1;i<n;i++)
        A+=Arie(pun[i].first,pun[i].second,pun[i+1].first,pun[i+1].second);

    A+=Arie(pun[n].first,pun[n].second,pun[1].first,pun[1].second);


    if(A>0)
        printf("%lf",A);
    else printf("%lf",-A);
    return 0;
}