#include <iostream>
#include <cstdio>

using namespace std;

struct punct{
    double x, y;
}vec[100001];

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int n;
    double a=0;
    cin>>n;
    for(int i=0; i<n; i++)
        cin>>vec[i].x>>vec[i].y;
    vec[n]=vec[0];
    for(int i=0; i<n; ++i){
        a+=(vec[i].x * vec[i + 1].y - vec[i+1].x * vec[i].y);
    }
    printf ("%lf",(a / 2.0));
    return 0;
}