#include <stdio.h>
#include <math.h>

#define N_MAX 100001

double x[ N_MAX ], y[ N_MAX ];

int main( ) {
    FILE * fin, * fout;
    fin = fopen( "aria.in", "r" );
    fout = fopen( "aria.out", "w" );

    int N;
    fscanf( fin, "%d", &N );

    double area = 0;
    int i;
    for( i = 0; i < N; i ++ ) {
        fscanf( fin, "%lf %lf", x + i, y + i );
    }
    x[ N ] = x[ 0 ];
    y[ N ] = y[ 0 ];

    for( i = 0; i < N; i ++ ) {
        area += ( x[ i ] * y[ i + 1 ] - x[ i + 1 ] * y[ i ] ) / 2;
    }

    fprintf( fout, "%lf", fabs( area ) );

    fclose( fin );
    fclose( fout );
}