#include <fstream>
#include <iomanip>

#define NMAX 100005

using namespace std;

ofstream g("aria.out") ;

struct elem{double x , y ;} v[NMAX] ;
int n ;
long double sol ;

long double solve(int a , int b){
    return v[a].x * v[b].y - v[b].x * v[a].y ;
}

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

    scanf("%d" , &n) ; //f >> n ;

    for(int i = 1 ; i <= n ; ++i){
        scanf("%lf %lf" , &v[i].x , &v[i].y) ; //f >> v[i].x >> v[i].y ;
    }

    for(int i = 1 ; i < n ; ++i){
        sol = sol + solve(i , i + 1) ;
    }

    sol = sol + solve(n , 1) ;

    if(sol < 0){
        sol = -sol ;
    }

    sol /= 2 ;

    g << setprecision(7) << fixed << sol ;
    return 0;
}