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

using namespace std;

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

int main()
{
    ifstream fi("aria.in");
    freopen("aria.out", "w", stdout);
    fi >> n;
    for(i = 1; i <= n; i++)
    {
        fi >> x[i] >> y[i];
        if(i > 1) S += (double)x[i]*y[i-1] - (double)x[i-1]*y[i];
    }
    S += (double)x[1]*y[n] - (double)y[1]*x[n];
    S = (S<0) ? -S : S;
    printf ("%lf", (S / 2.0));
    return 0;
}