import { readFileSync } from 'fs';

function Main(input: string) {
  const data = input.split('\n');

  const numbers = data[0].split(' ').map(Number);

  const res = numbers.find((value, index) => value !== index + 1);
  if (res == null) {
    throw new Error('Not Found');
  }

  console.log(res - 1);
}

Main(readFileSync('/dev/stdin', 'utf8'));