#include<fstream>
#include<cmath>
std::ifstream fin("aria.in");
std::ofstream fout("aria.out");
struct point{
    long double x, y;
}vec[100005];
int main()
{
    int n;
    fin>>n;
    for(int i=0; i<n; ++i)
        fin>>vec[i].x>>vec[i].y;
    vec[n]=vec[0];
    long double arie=0;
    for(int i=0; i<n; ++i)
        arie+=(vec[i].x*vec[i+1].y-vec[i+1].x*vec[i].y);
    fout<<fabs(arie/2.0);
    return 0;
}