#include <iostream>
#include <cstdio>
#include <algorithm>

#define Point pair<double,double>

using namespace std;

Point v[100005], O;
int N;


double det(Point A, Point B, Point C)
{
    return (A.first-C.first)*(B.second-C.second) - (A.second-C.second)*(B.first-C.first);
}

bool cmp(Point A, Point B)
{
    return det(v[1],A,B)>0;
}

void citire()
{
    freopen("aria.in","r",stdin);
    scanf("%d",&N);
    int pos = 1;
    for(int i=1; i<=N; i++)
    {
        scanf("%lf%lf",&v[i].first,&v[i].second);
        if(v[pos]>v[i])
        {
            pos = i;
        }

    }
    //swap(v[1],v[pos]);
}

void solve()
{
    double Arie=0;
    //sort(v+1,v+N+1,cmp);
    v[N+1] = v[1];
    for(int i=1; i<=N; i++)
        Arie = Arie + (v[i].first * v[i+1].second - v[i+1].first * v[i].second);
    Arie = Arie/2;
    freopen("aria.out","w",stdout);
    printf("%lf",Arie);
}

int main()
{
    citire();
    solve();
    return 0;
}