#include <bits/stdc++.h>
using namespace std;
const int MAXN = 100000;
struct point {
    long double x, y;
} v[MAXN];
int main()
{
    FILE *fin, *fout;
    int n, i;
    long double a;
    fin = fopen("aria.in", "r");
    fscanf(fin, "%d", &n);
    for (i = 0; i < n; i++) {
        fscanf(fin, "%Lf%Lf", &v[i].x, &v[i].y);
    }
    fclose(fin);
    a = v[n - 1].x * v[0].y - v[n - 1].y * v[0].x;
    for (i = 0; i < n - 1; i++) {
        a += v[i].x * v[i + 1].y - v[i].y * v[i + 1].x;
    }
    a /= 2;
    fout = fopen("aria.out", "w");
    fprintf(fout, "%.5Lf\n", a);
    fclose(fout);
    return 0;
}