"use strict";
const rl=require("readline").createInterface(process.stdin);

const g=function*(){
	var B=(yield rl.once("line",s=>g.next(s))).split(/ /).map(s=>parseInt(s,10));
	var A=[...Array(10).keys()].map(i=>i+1);
	console.log(A.filter(i=>!B.some(n=>i==n))[0]);
	process.exit();
}();
g.next();