#include <fstream>
#include <iomanip>
#include <cmath>

using namespace std;

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

double x[100005],y[100005];
double sol;
int n, i, j;

int main() {
    f >> n;
    for (i = 0; i < n; i++)
        f >> x[i]>>y[i];
    for(i = 0; i < n; i++)
        sol += (x[i]*y[(i+1)%n]-y[i]*x[(i+1)%n]);
    g <<fixed << setprecision(5)<< fabs(sol)/2;
    return 0;
}