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

using namespace std;

const int MAXN = 100010;
double x[MAXN], y[MAXN], solution = 0;
int n;

int main() {
    FILE *input, *output;
    input = fopen("aria.in", "r");
    output = fopen("aria.out", "w");
    fscanf(input, "%d", &n);
    for (int i = 0; i < n; i++)
        fscanf(input, "%lf%lf", &x[i], &y[i]);
    for (int i = 0; i < n - 1; i++)
        solution += ((x[i] * y[(i + 1)]) - (x[(i+1)] * y[i]));
    solution += ((x[n - 1] * y[0]) - (x[0] * y[n - 1]));
    solution = abs(solution / 2);
    fprintf(output, "%.5f", solution);
    return 0;
}