#include<cstdio>
    using namespace std;
    #define MAX 100001
    int N;
    struct punct
    {
        double x , y ;
    }P[MAX];
    double aria;

    void citire();
    void solve();
    void tipar();

    int main()
    {
        citire();
        solve();
        tipar();
        return 0;
    }

    void citire()
    {
        freopen("aria.in" , "r" , stdin );
        scanf("%d" , &N );
        for( int i = 1 ; i <= N ; ++i )
            scanf("%lf%lf" , &P[i].x , &P[i].y);
    }

    void solve()
    {
        for(int i = 1 ; i < N ; ++i )
            aria+=(P[i].x*P[i+1].y-P[i].y*P[i+1].x);
        aria+=(P[N].x*P[1].y-P[N].y*P[1].x);
        aria/=2;
    }

    void tipar()
    {
        freopen("aria.out" , "w" , stdout );
        printf("%.5lf" , aria );
    }