#include <bits/stdc++.h>
using namespace std;
int main()
{
    FILE *fin, *fout;
    int n, i;
    double a, px, py, x, y, ulx, uly;
    fin = fopen("dfs.in", "r");
    fscanf(fin, "%d%lf%lf", &n, &px, &py);
    a = 0;
    ulx = px;
    uly = py;
    for (i = 1; i < n; i++) {
        fscanf(fin, "%lf%lf", &x, &y);
        a += ulx * y - uly * x;
        ulx = x;
        uly = y;
    }
    fclose(fin);
    a += ulx * py - uly * px;
    a /= 2;
    fout = fopen("dfs.out", "w");
    fprintf(fout, "%.5lf\n", a);
    fclose(fout);
    return 0;
}