#include <iostream>
#include <cstdio>

using namespace std;
int n;
float a;
float x1,y1,x2,y2,xa,ya;
float arie()
{
    return (-x2*y1+x1*y2)/2;
}
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d\n", &n);
    scanf("%f %f", &x1, &y1);
    xa=x1, ya=y1;
    for(int i=1;i<n;i++)
    {
        scanf("%f %f", &x2, &y2);
        a+=arie();
        x1=x2, y1=y2;
    }
    x2=xa, y2=ya;
    a+=arie();
    printf("%.5f", a);
    return 0;
}