function Main (input){
  input = input.split("\n");
  tmp = input[0].split(" ");
  var a = parseInt(tmp[0],10);
  var b = parseInt(tmp[1],10);
  var x = parseInt(tmp[2],10);
  var kakudo = 0;
  var deg = 0;
  var tri = (a*a*b)/2
  if(x > tri){
    deg = 2*((b/a)-(x/(a*a*a)));
    kakudo = deg;
  }else if(x <= tri){
    deg = (a*b*b)/(2*x);
    kakudo = deg;
  }else{
    "";  
  }
  console.log(kakudo);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));