#include <cstdio>
#include <cmath>
const int MAX = 21;
typedef double D;
 
D dis(D x1,D y1,D x2,D y2){
  D d;
  d = sqrt(pow(x2-x1,2)+pow(y2-y1,2));
  return d;
}
 
int main(){
  D x[MAX],y[MAX];
  int t = 0;
  D S = 0.0,z,a,b,c;
 
  while(true){
    if(scanf("%lf,%lf",&x[t] ,&y[t]) == EOF) break;
    t++;
  }
 
  for(int i = 2 ; i < t ; i++){
    a = dis(x[0],y[0],x[i-1],y[i-1]);
    b = dis(x[0],y[0],x[i],y[i]);
    c = dis(x[i-1],y[i-1],x[i],y[i]);
    z = (a + b + c) / 2;
 
    S += sqrt(z*(z-a)*(z-b)*(z-c));
  }
  
  printf("%.6f\n",S);
 
  return 0;
}