#include <iostream>
#include <iomanip>

#define NMAX 100005

using namespace std;

typedef long double ld;

struct elem {
    ld x, y;
} a[NMAX];

int n;

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    cin >> n;
    for(int i = 1; i <= n; ++i)
        cin >> a[i].x >> a[i].y;
    a[0] = a[n];
    ld ans = 0;
    for(int i = 1; i <= n; ++i)
        ans += a[i - 1].x * a[i].y - a[i].x * a[i - 1].y;
    ans /= 2.;
    cout << fixed << setprecision(8) << (ans > 0 ? ans : -ans);
    return 0;
}