#include <cstdio>
const int NMAX = 100000;
using namespace std;
double x[NMAX+5], y[NMAX+5];
int main() {
    freopen ( "arie.in", "r", stdin );
    freopen ( "arie.out", "w", stdout );
    int n, i;
    double a;
    scanf ( "%d", &n );
    for ( i = 1 ; i <= n ; ++ i ) {
        scanf ( "%lf", &x[i] );
        scanf ( "%lf", &y[i] );
    }
    x[0] = x[n];
    y[0] = y[n];
    a = 0.0;
    for ( i = 1 ; i <= n ; ++ i )
        a += ( x[i] * y[(i+1)%n] ) - ( y[i] * x[(i+1)%n] );
    a /= 2.0;
    printf ( "%lf", a );
    return 0;
}