#include <bits/stdc++.h>
#define rep(i,l,n) for(int i=l;i<n;i++)
#define rer(i,l,n) for(int i=l;i<=n;i++)
#define all(a) a.begin(),a.end()
#define o(a) cout<<a<<endl
#define pb(a) push_back(a)
#define mk(a,b) make_pair(a,b)
#define fi first
#define se second
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pii;

int main(){
	int c=0;
	double  a,b,sum=0.0;
	int x[21]={},y[21]={};
	while(scanf("%lf,%lf",&a,&b)!=EOF){
		if(c==0){
			x[c]=a; y[c]=b;
		}else{
			x[c]=a-x[0]; y[c]=b-y[0];
		}
		c++;
	}
	rep(i,1,c-1){
		sum+=0.5*abs(x[i]*y[i+1]-x[i+1]*y[i]);
	}
	printf("%.8lf\n",sum);
}