"use strict";
var input=require("fs").readFileSync("/dev/stdin","utf8");
var cin=input.split(/ |\n/),cid=0;
function next(){return +cin[cid++];}
function nextstr(){return cin[cid++];}
function nextbig(){return BigInt(cin[cid++]);}
function nexts(n,a){return a?cin.slice(cid,cid+=n):cin.slice(cid,cid+=n).map(a=>+a);}
function nextm(h,w,a){var r=[],i=0;if(a)for(;i<h;i++)r.push(cin.slice(cid,cid+=w));else for(;i<h;i++)r.push(cin.slice(cid,cid+=w).map(a=>+a));return r;}
function xArray(v){var a=arguments,l=a.length,r="Array(a["+--l+"]).fill().map(x=>{return "+v+";})";while(--l)r="Array(a["+l+"]).fill().map(x=>"+r+")";return eval(r);}

var mod = 1e9+7;
function mul(){for(var a=arguments,r=a[0],i=a.length;--i;)r=((r>>16)*a[i]%mod*65536+(r&65535)*a[i])%mod;return r;}
var fac=[],finv=[],invs=[],pow2=[];
function fset(n){
  fac.length=n+1,fac[0]=fac[1]=1;
  for(var i=2;i<=n;)fac[i]=mul(fac[i-1],i++);
  finv.length=n+1,finv[0]=finv[1]=1,finv[n]=inv(fac[n]);
  for(i=n;2<i;)finv[i-1]=mul(finv[i],i--);
  //invs.length=n+1,invs[0]=invs[1]=1;
  //for(i=2;i<=n;)invs[i]=mul(fac[i-1],finv[i++]);
  //pow2.length=n+1,pow2[0]=1;
  //for(i=1;i<=n;i++)pow2[i]=pow2[i-1]*2%mod;
}
function inv(b){for(var a=mod,u=0,v=1,t;b;v=t)t=a/b|0,a-=t*b,u-=t*v,t=a,a=b,b=t,t=u,u=v;u%=mod;return u<0?u+mod:u;}
function pow(a,n){for(var r=1;n;a=mul(a,a),n>>=1)if(n&1)r=mul(a,r);return r;}
function ncr(n,r){return mul(fac[n],finv[r],finv[n-r]);}
function npr(n,r){return mul(fac[n],finv[n-r]);}
function mput(n){return (n%mod+mod)%mod;}

var tm = +(new Date()) + 1900;
var myOut = main();
if(myOut !== undefined)console.log(String(myOut));

function main(){
  var n = next();
  var a1 = nextbig(), a2 = nextbig();
  var x = 0n;
  for(var i = 2; i < n; i++)x ^= nextbig();
  var ans = 0n, a = a1^a2^x;
  while(a){
    if(a1 === 1 || new Date() > tm)return -1;
    for(var i = 40; i >= 0; i--){
      var k = 1n << BigInt(i);
      if((a & k) === 0n)continue;
      var u;
      u = (a1 & (k-1n)) + 1n;
      u = min(u, k-(a2&(k-1n)) );
      if(u < a1){
        ans += u, a1 -= u, a2 += u;
        break;
      }else return -1;
    }
    a = a1^a2^x;
  }
  return ans;
}

function min(a,b){
  return a > b ? b : a;
}