#include<bits/stdc++.h>
#define newline '\n'
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
///**************************

typedef long double lodo;
const int NMAX = 1e5 + 3;
struct Point
{
    lodo x, y;
} p[NMAX];

int n;
lodo area;

int main()
{
    fin >> n;
    for (int i = 1; i <= n; i++)
        fin >> p[i].x >> p[i].y;

    p[n + 1] = p[1];
    for (int i = 1; i <= n; i++)
        area += p[i].x * p[i + 1].y - p[i + 1].x * p[i].y;

    area /= 2.0;
    fout << fixed << showpoint << setprecision(5) << area << newline;
    return 0;
}