#include <iostream>
#include <stdio.h>
#include <algorithm>
using namespace std;
int x[100010],y[100010],t;

void cit()
 {
    scanf("%d",&t);
    for(int i = 0 ; i <  t ; i ++)
        scanf("%d %d",&x[i],&y[i]);
}

int det(int x1,int x2, int y1 ,int y2)
{
    int m = x1 - x2;
    int n = y1 + y2;
    return m*n/2;
}

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    cit();
    long long p =0;
    for(int i = 0 ; i < t -1 ; i++)
        p += det(x[i],x[i+1],y[i],y[i+1]);
    printf("%lld",abs(p));

    return 0;
}