#include <bits/stdc++.h>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

int n;
double aria;
struct {double x,y;} p[1<<17];

int main()
{   f>>n;
    for(int i=1; i<=n; i++)
        f>>p[i].x>>p[i].y;
    p[n+1]=p[1];
    for(int i=1; i<=n; i++)
        aria+=(p[i].x*p[i+1].y-p[i+1].x*p[i].y);
    g<<fabs(aria/2.0);
    return 0;
}