
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <ctime>
#include <cassert>
#include <iostream>
#include <cctype>
#include <sstream>
#include <string>
#include <list>
#include <vector>
#include <queue>
#include <set>
#include <stack>
#include <map>
#include <utility>
#include <numeric>
#include <algorithm>
#include <iterator>
#include <bitset>
#include <complex>
#include <fstream>
#include <iomanip>
using namespace std;
typedef long long ll;
const double EPS = 1e-9;
typedef vector<int> vint;
typedef pair<int, int> pint;
#define rep(i, n) REP(i, 0, n)
#define ALL(v) v.begin(), v.end()
#define MSG(a) cout << #a << " " << a << endl;
#define REP(i, x, n) for(int i = x; i < n; i++)
template<class T> T RoundOff(T a){ return int(a+.5-(a<0)); }
template<class T, class C> void chmax(T& a, C b){ if(a < b) a = b; }
template<class T, class C> void chmin(T& a, C b){ if(b < a) a = b; }
template<class T, class C> pair<T, C> mp(T a, C b){ return make_pair(a, b); }


struct P{ double x, y; };
bool operator > (P x, P y)
{ return x.x > y.x || (x.x == y.x && x.y > y.x); }

double CalcArea(P a, P b, P c)
{
	double A = sqrt(pow(a.x - b.x, 2.0) + pow(a.y - b.y, 2.0));
	double B = sqrt(pow(b.x - c.x, 2.0) + pow(b.y - c.y, 2.0));
	double C = sqrt(pow(c.x - a.x, 2.0) + pow(c.y - a.y, 2.0));
    
	double s = (A + B + C) / 2;
    
	return sqrt(s * (s - A) * (s - B) * (s - C));
}


int main()
{
	vector<P> points;

	P in;
	char k;
	while(cin >> in.x >> k >> in.y && in.x != -100)
		points.push_back(in);

	double S = 0;
	REP(i, 1, points.size()-1)
		S += CalcArea(points[0], points[i], points[i + 1]);

	cout << setprecision(10) << S << endl;
}