#include <iostream>
#include<stdio.h>

int x[100001], y[100001];

using namespace std;

int main() {
    FILE *fin, *fout;
    int n, i, a;

    fin=fopen("aria.in", "r");
    fout=fopen("aria.out", "w");
    fscanf( fin, "%d", &n);
    for(i=1;i<=n;i++) {
       fscanf( fin, "%d %d", &x[i], &y[i]);
    }
    a=0;
    for(i=1;i<=n;i++) {
            if(i==n) {
                    y[i+1]=y[1];
            x[i+1]=x[1];
            }
            a=a+(x[i]*y[i+1]-x[i+1]*y[i]);
    }
    a=a/2;
    fprintf( fout, "%d", a);
    fclose( fin );
    fclose( fout );
    return 0;
}