#include <fstream>
#include<iomanip>
using namespace std;

struct Punct{
    double x,y;
};

const int N_MAX = 100000;

double mod(double x){
    if(x>0) return x;
    return -x;
}

int main() {

    ifstream f("aria.in");
    ofstream g("aria.out");
    int n;
    Punct puncte[N_MAX];

    f>>n;
    for(int i=0;i<n;++i){
        f>>puncte[i].x>>puncte[i].y;
    }
    double arie;
    arie=0;
    for(int i=0;i<n-1;++i){
        arie+=(puncte[i].y+puncte[i+1].y)/2*(puncte[i+1].x-puncte[i].x);
    }
    g<<setprecision(5)<<mod(arie);
    return 0;
}