#include <fstream>
#include <iomanip>
#include <cmath>
#define det(i,j) a[i]*b[j] - b[i]*a[j]

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

const int nmax = 100005;
double a[nmax], b[nmax], sol;
int n, i;

int main() {
    for (f >> n, i = 1; i <= n; i++) f >> a[i] >> b[i];
    a[n+1] = a[1], b[n+1] = b[1];
    for (i = 2; i <= n+1; i++) sol += det(i-1,i);
    g << fixed << setprecision(5) << fabs(sol)/2;
    return 0;
}