#include<fstream>
#include<iomanip>
using namespace std;
ifstream  fin("aria.in");
ofstream fout("aria.out");
int n;
double x0,y0,x1,y1,x2,y2,s;
int main(){
    int i;
    fin>>n;
    fin>>x0>>y0;
    s=0;
    x1=x0;y1=y0;
    for(i=2;i<=n;i++){
        fin>>x2>>y2;
        s=s+x1*y2-x2*y1;
        x1=x2;y1=y2;
    }
    s=s+x2*y0-x0*y2;
    s=s/2.0;
    if(s<0)s=-s;
    fout<<fixed<<setprecision(15)<<s;
    fout.close();
    fin.close();
    return 0;
}