#include <cstdio>

#include <iostream>

using namespace std;

float terulet(float x, float y, float x1, float y1, float x2, float y2){
    x1-=x;
    y1-=y;
    x2-=x;
    y2-=y;
    return x1*y2-x2*y1;
}

float abs(float d){
    return d>=0?d:-d;
}

int main()
{
    FILE* f=fopen("arie.in", "r");
    int n;
    fscanf(f, "%d", &n);

    float x, y, x1, y1, x2, y2;
    float ter=0;

    fscanf(f, "%f %f", &x, &y);
    fscanf(f, "%f %f", &x1, &y1);
    for(int i=2;i<n;i++){
        fscanf(f, "%f %f", &x2, &y2);
        ter+=terulet(x, y, x1, y1, x2, y2);
        x1=x2;
        y1=y2;
    }

    fclose(f);
    f=fopen("arie.out", "w");

    ter=abs(ter)/2;

    fprintf(f, "%f", ter);
    return 0;
}