var lines = [];
var reader = require('readline').createInterface({
  input: process.stdin,
  output: process.stdout
});
var counter = 0;
reader.on('line', function (line) {
  counter += 1;
  lines.push(line);
  if( counter == 2){
    main();
    process.exit(0);
  }
});
process.stdin.on('end', function () {
});

function main(){
  var n = parseInt(lines[0]);
  var xs = [];
  lines[1].split(" ").map(function(x) {
    xs.push(parseInt(x));
  });
  xs.sort(function(a,b){
    if(a < b) return -1;
    if(a > b) return 1;
    return 0;
  });
  var sa = xs[1] - xs[0];
  if(sa == 0){
    console.log("NO");
    return 0;
  }
  for(var i = 1; i < n - 1; ++i){
    if(sa != xs[i + 1] - xs[i]){
      console.log("NO");
      return 0;
    }else{
      console.log("YES");
      return 0;
    }
  }
}