#include <cstdio>
#include <algorithm>
#define NMax 100000
#define x first
#define y second
using namespace std;

typedef pair<double, double> Punct;
Punct v[NMax+1];

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

    int i,N;
    double res;

    scanf("%d",&N);
    for(i = 1; i <= N; ++i) scanf("%lf %lf", &v[i].x, &v[i].y);
    v[N+1] = v[1];

    for(res = 0, i = 1; i <= N; ++i) res = res + ( v[i].x*v[i+1].y - v[i+1].x*v[i].y );
    printf("%f\n", res/2);


return 0;
}