#include <iostream>
#include <cstdio>

using namespace std;


float aria(float x1,float y1,float x2,float y2,float x3,float y3);
float xi,yi;
int n;

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);

    if(n<=2)cout<<"0";
    else{

    float x1,x2,x3,y1,y2,y3;
    scanf("%f %f %f %f %f %f",&x1,&y1,&x2,&y2,&x3,&y3);

    float arie=0;
    arie+=aria(x1,y1,x2,y2,x3,y3);

    for(int i=4; i<=n; i++)
    {
        x2=x3;
        y2=y3;
        scanf("%f %f",&x3,&y3);
        arie+=aria(x1,y1,x2,y2,x3,y3);
    }



    printf("%f",arie);
    }

    return 0;
}

float aria(float x1,float y1,float x2,float y2,float x3,float y3)
{

    float s=0;

    s=x1*y2+x3*y1+x2*y3-x3*y2-x1*y3-x2*y1;
    if(s<0)s=-1*s;
    s=s/2;
    return s;
}