#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    FILE* in = fopen("aria.in", "r");
    FILE* out = fopen("aria.out", "w");

    int n;
    float ans = 0;
    fscanf(in, "%d", &n);
    
    float x1, y1, xp, yp, x, y;
    fscanf(in, "%f%f", &x1, &y1);
    xp = x1;
    yp = y1;

    for(int i = 1; i < n; i++) {
        fscanf(in, "%f%f", &x, &y);

        ans += xp * y - x * yp;

        xp = x;
        yp = y;
    }

    ans += x * y1 - x1 * y;

    ans /= 2;

    fprintf(out, "%.5ff\n", ans);

    return 0;

}