#include <cstdio>
using namespace std;

#define Nmax 100002

FILE *f = fopen ( "aria.in", "r" );
FILE *g = fopen ( "aria.out", "w" );

struct point{
    double x, y;
}v[Nmax];

double abs ( double x ){
    if ( x < 0 )
        x *= -1;
    return x;
}

int main(){

    int N;
    double arie = 0;

    fscanf ( f, "%d", &N );

    for ( int i = 1; i <= N; ++i )
        fscanf ( f, "%lf%lf", &v[i].x, &v[i].y );

    v[N+1] = v[1];

    for ( int i = 1; i <= N; ++i )
        arie = arie + ( v[i].x * v[i+1].y - v[i].y * v[i+1].x );

    fprintf ( g, "%lf", abs ( arie ) * 0.5 );

    return 0;
}