#include <cstdio>

using namespace std;

int abs( double k )
{
    if( k<0 )
        k=-k;

    return k;
}

int main()
{
    freopen( "aria.in", "r", stdin );
    freopen( "aria.out", "w", stdout );

    int n, i;
    double a=0, x, y, x1, y1, x2, y2;

    scanf( "%d%lf%lf", &n, &x, &y );

    x1=x;
    y1=y;

    for( i=2;i<=n;i++ )
    {
        scanf( "%lf%lf", &x2, &y2 );

        a=abs(a+(x1-x2)*(y1+y2));

        x1=x2;
        y1=y2;
    }

    a=abs(a+(x1-x)*(y1+y))/2;

    printf( "%.5lf", a );

    return 0;
}