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

int N;

struct punct
{
    double x,y;
}S[Nmax];

double Sol;

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&N);
    for(int i=0;i<N;++i)
        scanf("%lf %lf",&S[i].x,&S[i].y);
    S[N]=S[0];
    for(int i=0;i<N;++i)
        Sol+=(S[i].x*S[i+1].y-S[i+1].x*S[i].y);
    printf("%lf",abs(Sol/2.));
    return 0;
}