#include<fstream>
#include<iomanip>
using namespace std;
long double arie,x[100005],y[100005];
int i,n;

int main(void){
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    fin>>n;
    for (i=1; i<=n; ++i) fin>>x[i]>>y[i];
    x[n+1]=x[1]; y[n+1]=y[1];
    for (i=1; i<=n; ++i) arie+=(x[i]*y[i+1]-y[i]*x[i+1]);
    arie/=2;
    fout<<fixed<<setprecision(8)<<arie;
 return(0);
}