#include <iostream>
#include <fstream>
#define inf 2147483647

using namespace std;

ifstream f ("aria.in");
ofstream g ("aria.out");

double sum;
int n;

struct poligon{
    double x,y;
    double m;
}a[100001];

int main(){
    f>>n;
    for(int i=1; i<=n; ++i)
        f>>a[i].x>>a[i].y;
    if(n>2){
        for(int i=2; i<=n; ++i)
            sum=sum+(a[i-1].x*a[i].y-a[i].x*a[i-1].y)/2;
        sum=sum+(a[n].x*a[1].y-a[1].x*a[n].y)/2;
        if(sum<0)sum=sum*(-1);
        g<<sum<<'\n';
    }
    else g<<"0\n";
}