#include <fstream>

using namespace std;
struct ura{
    double x,y;
}v[100001];
double sol = 0;
ifstream cin("aria.in");
ofstream cout("aria.out");
void aria(){
    int n,i;
    cin >> n;
    for(i = 1;i <= n;i++){
        cin >> v[i].x >> v[i].y;
    }
    v[n + 1].x = v[1].x;
    v[n + 1].y = v[1].y;
    for(i = 1;i <= n;i++){
        sol += (v[i].x * v[i + 1].y - v[i+1].x * v[i].y);
    }
    sol /= 2;
    cout << sol;
}
int main()
{
    aria();
    return 0;
}