#include<stdio.h>
int x[100031], y[100031];
int main(){
    int n;
    scanf("%d", &n);
    for(int i = 1; i<=n; i++)
    {
        scanf("%d %d",&x[i], &y[i]);
    }
    x[n+1] = x[1];
    y[n+1] = y[1];
    long int sum = 0;
    for(int i = 2; i<=n + 1; i++)
    {
        sum += x[i] * y[i-1] - x[i-1] * y[i];
    }
    if(sum < 0) sum = -sum;
    printf("%ld", sum/2);
    return 0;
}