# include <iostream>
# include <fstream>
using namespace std;

# define MAXN 100010

ifstream f("aria.in");
FILE * g = fopen("aria.out", "w");

int n;
float x[MAXN], y[MAXN];
float aria = 0;

int main()
{
    f >> n;
    f >> x[1] >> y[1];
    for (int i = 2; i <= n; i++) {
        f >> x[i] >> y[i];
        aria += (x[i - 1] * y[i] - x[i] * y[i - 1]);
    }
    aria += (x[n] * y[1] - x[1] * y[n]);

    fprintf(g, "%f", aria / 2);

    f.close();
    fclose(g);
    return 0;
}