let input = require('fs').readFileSync('/dev/stdin', 'utf8');
let [[N], x] = input.split('\n').map(v => v.split(/\s/).map(v => Number(v)));
if ([...new Set(x)].length != x.length) {
  console.log('NO');
} else {
  x.sort((a, b) => a - b);
  let diff = 0;
  for (let i = 1; i < x.length; i++) {
    if (i == 1) diff = x[i] - x[i - 1];
    else if (i == x.length - 1) console.log('YES');
    else if (diff != x[i] - x[i - 1]) {
      console.log('NO');
      break;
    }
  }
}