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

using namespace std;

#define vec vector
#define rep(i, n) for (int i = 0; i < (int)n; i++)
#define pb push_back

typedef struct
{
	double x;
	double y;
} P;

int ret(int i)
{
	if (i < 0) return -i;
	else return i;
}

int main(void)
{
	double ans = 0;
	P z0, z1, z2;
	cin >> z0.x >> z0.y;
	cin >> z1.x >> z1.y;
	z1.x -= z0.x;
	z1.y -= z0.y;
	char get;
	while(cin >> z2.x >> get >> z2.y) {
		z2.x -= z0.x;
		z2.y -= z0.y;
		ans += z1.x * z2.y - z1.y * z2.x;
		z1 = z2;
	}
	cout << ret(ans) << endl;
	return 0;
}