#include <fstream>

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

struct punct{
    double x, y;
} v[100010];


int n;
double aria;
int main()
{
    fin >> n;
    for(int i = 1; i <= n; ++i){
        fin >> v[i].x >> v[i].y;
    }

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