#include <iostream>
#include <cstdio>
#define Dmax 100010
using namespace std;

struct COORDONATE {double x, y;}v[Dmax]; int N; double SUMA;

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

    cin >> N;

    for(int i=1; i<=N; i++)
        cin>>v[i].x>>v[i].y;

    v[N+1].x=v[1].x; v[N+1].y=v[1].y; //CONSIDERAM PUNCTUL A(i)=A(N+1)

    for(int i=1; i<=N; i++)
        SUMA=SUMA+((v[i].x*v[i+1].y)-(v[i+1].x*v[i].y));

    SUMA=SUMA/2;

    printf("%.6f",SUMA);

    return 0;
}