#include <bits/stdc++.h>

using namespace std;

int main()
{
    FILE *f = fopen("aria.in", "r");
    FILE *g = fopen("aria.out", "w");
    int N;
    double A = 0;
    struct punct{
        double x, y;
    } v[100005];

    fscanf(f, "%d", &N);
    for(int i = 0; i < N; i ++) {
        fscanf(f, "%lf %lf", &v[i].x, &v[i].y);
    }

    v[N] = v[0];
    for(int i = 0; i < N; i ++)
        A += (v[i].x * v[i + 1].y - v[i + 1].x * v[i].y);

    A /= 2;
    fprintf(g, "%.9f\n", A);
    return 0;
}