#include<cstdio>
#include<algorithm>
using namespace std;

pair <double,double> v[100024];
int N;
double A;


double Area(double X1, double X2, double X3, double Y1, double Y2, double Y3){

    return ((X3-X1)*(Y2-Y1) - (X2-X1)*(Y3-Y1));
}


int main(){

    freopen("aria.in","r", stdin);
    freopen("aria.out","w", stdout);

    scanf("%d", &N);

    for(int i=1;i<=N;i++){
        scanf("%lf %lf", &v[i].first, &v[i].second);
    }

    v[++N]=v[1];
    for(int i=1;i<=N-1;i++){
        A+=Area(0, v[i].first, v[i+1].first, 0, v[i].second, v[i+1].second);
    }

    printf("%.5f", A/2);
    fclose(stdin);
    fclose(stdout);

    return 0;
}