#include <fstream>
#include <cmath>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

struct loc
{
    double x, y;
} v[100001];

double sum;
int n;

int main()
{
   
    fin>>n;
    
    for(int i = 1; i <= n; ++i)
    {
       fin>>v[i].x>>v[i].y;
    }
    v[n + 1].x = v[1].x;
    v[n + 1].y = v[1].y;
    for(int i = 1; i <= n; ++i)
    {
        sum+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    }
    fout<<abs(sum)/2.0;
    
    return 0;
}