
var input = require("fs").readFileSync("/dev/stdin", "utf8");
var data = input.split(' ');
var n = Number.parseInt(data[0]);
var a = Number.parseInt(data[data[1]);
var b = Number.parseInt(data[data[2]);

var pos = b-a+1
var nums = n-2

var fact = function(n){
	var p = 1;
  while(n>1) p *= n--;
  return p;
}

//console.log(n,a,b);
var sume = 0;
for(var i=1; i<=nums; i++ ){
	sume += fact(pos) / (fact(i) * fact(pos-i));
}
if( a>b )
	console.log(0);
else
	console.log(sume);
//*/