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


    double det(punct a , punct b)
    {
        return a.x*b.y-b.x*a.y;
    }

    int main()
    {
        freopen("aria.in" , "r" , stdin );
        scanf("%d" , &N );
        for(int i = 1 ; i<= N ; ++i )
            scanf("%lf%lf" , &P[i].x , &P[i].y);
        P[N+1] = P[1];
        for(int i = 1 ; i <= N ; ++i )
            A+=det(P[i],P[i+1]);
        A/=2;
        freopen("aria.out" , "w" , stdout );
        printf("%lf" , A);
        return 0;
    }