#include <fstream>
#include <iomanip>
using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

struct p{long double x,y;}v[100010];

long double sol;
long double abbs(long double a){if(a >= 0) return a; return a*-1.0;}
int main(){
    int n;
    f >> n;
    for(int i = 0; i < n; ++ i){
        f >> v[i].x >> v[i].y;
    }
    v[n] = v[0];
    for(int i = 0; i <n ;++ i){
        sol += v[i].x*v[i+1].y - v[i].y*v[i+1].x;
    }
    g << setprecision(5) << fixed << abbs(sol)/2.0;
    return 0;
}