#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct varf{
    int x,y;
}v[100001];
int n;
long long A,ti;
int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for(int i=1;i<=n;++i)
        A+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    fout<<(A/2.0)<<endl;
    return 0;
}