#include <cstdio>
using namespace std;

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

#define Nmax 100002

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

int main(){
    int N;
    double area = 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 )
        area += ( v[i].x * v[i+1].y - v[i+1].x * v[i].y );

    area *= 0.5;

    fprintf ( g, "%.5lf", area );

    return 0;
}