#include <bits/stdc++.h>
#define maxN 100002
using namespace std;
double A;
struct point
{
    double x, y;
}v[maxN];
int n;
void read()
{
    int i;
    freopen("aria.in", "r", stdin);
    scanf("%d", &n);
    for (i = 1; i <= n; ++ i)
        scanf("%lf %lf", &v[i].x, &v[i].y);
    v[n + 1] = v[1];
}
void solve()
{
    int i;
    for (i = 1; i <= n; ++ i)
        A += v[i].x * v[i + 1].y - v[i + 1].x * v[i].y;
    if (A < 0)
        A = -A;
}
void write()
{
    freopen("aria.out", "w", stdout);
    A = A / 2.00000;
    printf("%.5lf", A);
}
int main()
{
    read();
    solve();
    write();
    return 0;
}