#include<stdio.h>
long double x[100031], y[100031];
int main(){
    FILE *in = fopen("aria.in", "r");
    FILE *out = fopen("aria.out", "w");
    
    int n;
    fscanf(in, "%d", &n);
    for(int i = 1; i<=n; i++)
    {
        fscanf(in, "%Lf %Lf",&x[i], &y[i]);
    }
    x[n+1] = x[1];
    y[n+1] = y[1];
    long double 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;
    fprintf(out, "%Lf", sum/2);
    fclose(in);
    fclose(out);
    return 0;
}