#include <fstream>
using namespace std;

ifstream cin("aria.in");
ofstream cout("aria.out");

int main()
{
    long double a,b,c,d,x,y,rasp=0;
    int n,i;
    cin>>n>>a>>b;c=a;d=b;
    for(i=2;i<=n;i++){
        cin>>x>>y;
        rasp+=x*b-y*a;
        a=x;
        b=y;
    }
    rasp+=c*b-d*a;
    if(rasp<0) rasp=-rasp;
    cout.precision(5);
    cout<<fixed<<rasp/2;
    return 0;
}