#include <fstream>
#include <iomanip>
#define N 100010

using namespace std;

double x[N], y[N], S;
int n, i;

int main()
{
    ifstream fi("aria.in");
    ofstream fo("aria.out");
    fi >> n;
    for(i = 1; i <= n; i++)
    {
        fi >> x[i] >> y[i];
        if(i > 1) S += (double)(x[i]*y[i-1] - x[i-1]*y[i]);
    }
    S += x[1]*y[n] - y[1]*x[n];
    S = (S > 0) ? S : -S;
    fo << setprecision(6) << fixed;
    fo << S/2 << "\n";
    return 0;
}