#include <bits/stdc++.h>

using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
int n;
double ans = 0;
struct cell
{
    int x, y;
}c[100010];
int main()
{
    in >> n;
    for(int i = 1;i <= n;i++)
        in >> c[i].x >> c[i].y;
    c[n + 1].x = c[1].x;
    c[n + 1].y = c[1].y;
    for(int i = 1;i <= n;i++)
        ans = ans + (1.0 * (1.0 * c[i].x * c[i + 1].y - 1.0 * c[i + 1].x * c[i].y)) / 2.0;
    out << ans;
    return 0;
}