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

#define Nmax 100002

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

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

int main(){

    int N, arie = 0;

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

    for ( int i = 1; i <= N; ++i )
        fscanf ( f, "%d%d", &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, "%d", abs ( arie ) >> 1 );

    return 0;
}