#include<cstdio>
#include<string>

using namespace std;

#ifdef HOME
const string inputFile = "input.txt";
const string outputFile = "output.txt";
#else
const string problemName = "aria";
const string inputFile = problemName + ".in";
const string outputFile = problemName + ".out";
#endif

const int NMAX = 100000 + 5;

int N;
double X[NMAX];
double Y[NMAX];
double arie;

double cp(int i, int j) {
    return (X[i] * 1.0 * Y[j] - X[j] * 1.0 * Y[i]);
}

int main() {
    int i;

    freopen(inputFile.c_str(), "r", stdin);
    freopen(outputFile.c_str(), "w", stdout);

    scanf("%d", &N);

    for(i = 1; i <= N; i++)
        scanf("%lf%lf", &X[i], &Y[i]);

    X[N + 1] = X[1];
    Y[N + 1] = Y[1];

    for(i = 1; i <= N; i++)
        arie += cp(i, i + 1);

    arie *= 0.5;

    printf("%.5f\n", arie);

    return 0;
}