#include<fstream>
#define DMAX 100005
using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

struct pct{
    int x, y;
};

pct v[DMAX];

double abs(double x){
    if(x >= 0)
        return x;
    else return -x;
}

int main(){
    int n;
    fin>>n;
    for(int i=0; i<n; i++){
        fin>>v[i].x>>v[i].y;
    }
    v[n].x = v[0].x;
    v[n].y = v[0].y;

    int S = 0;

    for(int i=0; i<n; i++){
        S += v[i].x * v[i+1].y - v[i+1].x * v[i].y;
    }
    fout<<abs(double(S)/2);
    return 0;
}