
var data = readline().split(' ');
let n=data[0];
let a=data[1];
let b=data[2];

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

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

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