Language: C++

void MAIN() {
	cin>>n;
	vll A(n+1);
	for(ll i=1;i<=n;i++) cin>>A[i];

	ll XOR = 0,sum = A[1] + A[2];
	for(ll i=3;i<=n;i++) XOR^=A[i];

	ll val = sum - XOR;
	
	if(val & 1 || sum < XOR) {
		cout<<-1;
		return;
	}

	ll AND = val/2;
	ll a =0,b=0;

	for(ll i=0;i<=39;i++) {
		ll x = (1ll << i);
        ll xx = XOR & x;
        ll yy = AND & x;
        
        if(xx == x && yy == x) {
        	cout<<-1;
        	return;
        }
        else if(xx == 0) {
        	a += yy;
        	b += yy;
        }
        else {
        	b += x;
        }
	}

	for (ll i = 39; i >= 0; i--){
        ll x = (1ll << i);
        if(((b&x) == x) && ((a&x) == 0)){
            if(b-x >= A[2]) b -= x, a += x;
        }
    }		

    if(a && b >= A[2])
    	cout<<A[1] - a;
    else cout<<-1;newl;
}

Compiler output:
a.cc: In function 'void MAIN()':
a.cc:2:9: error: 'cin' was not declared in this scope
    2 |         cin>>n;
      |         ^~~
a.cc:2:14: error: 'n' was not declared in this scope
    2 |         cin>>n;
      |              ^
a.cc:3:9: error: 'vll' was not declared in this scope
    3 |         vll A(n+1);
      |         ^~~
a.cc:4:13: error: 'll' was not declared in this scope
    4 |         for(ll i=1;i<=n;i++) cin>>A[i];
      |             ^~
a.cc:4:20: error: 'i' was not declared in this scope
    4 |         for(ll i=1;i<=n;i++) cin>>A[i];
      |                    ^
a.cc:4:35: error: 'A' was not declared in this scope
    4 |         for(ll i=1;i<=n;i++) cin>>A[i];
      |                                   ^
a.cc:6:9: error: 'll' was not declared in this scope
    6 |         ll XOR = 0,sum = A[1] + A[2];
      |         ^~
a.cc:7:15: error: expected ';' before 'i'
    7 |         for(ll i=3;i<=n;i++) XOR^=A[i];
      |               ^~
      |               ;
a.cc:7:20: error: 'i' was not declared in this scope
    7 |         for(ll i=3;i<=n;i++) XOR^=A[i];
      |                    ^
a.cc:7:30: error: 'XOR' was not declared in this scope
    7 |         for(ll i=3;i<=n;i++) XOR^=A[i];
      |                              ^~~
a.cc:7:35: error: 'A' was not declared in this scope
    7 |         for(ll i=3;i<=n;i++) XOR^=A[i];
      |                                   ^
a.cc:9:11: error: expected ';' before 'val'
    9 |         ll val = sum - XOR;
      |           ^~~~
      |           ;
a.cc:11:12: error: 'val' was not declared in this scope
   11 |         if(val & 1 || sum < XOR) {
      |            ^~~
a.cc:11:23: error: 'sum' was not declared in this scope
   11 |         if(val & 1 || sum < XOR) {
      |                       ^~~
a.cc:11:29: error: 'XOR' was not declared in this scope
   11 |         if(val & 1 || sum < XOR) {
      |                             ^~~
a.cc:12:17: error: 'cout' was not declared in this scope
   12 |                 cout<<-1;
      |                 ^~~~
a.cc:16:11: error: expected ';' before 'AND'
   16 |         ll AND = val/2;
      |           ^~~~
      |           ;
a.cc:17:11: error: expected ';' before 'a'
   17 |         ll a =0,b=0;
      |           ^~
      |           ;
a.cc:19:15: error: expected ';' before 'i'
   19 |         for(ll i=0;i<=39;i++) {
      |               ^~
      |               ;
a.cc:19:20: error: 'i' was not declared in this scope
   19 |         for(ll i=0;i<=39;i++) {
      |                    ^
a.cc:20:19: error: expected ';' before 'x'
   20 |                 ll x = (1ll << i);
      |                   ^~
      |                   ;
a.cc:21:11: error: expected ';' before 'xx'
   21 |         ll xx = XOR & x;
      |           ^~~
      |           ;
a.cc:22:11: error: expected ';' before 'yy'
   22 |         ll yy = AND & x;
      |           ^~~
      |           ;
a.cc:24:12: error: 'xx' was not declared in this scope
   24 |         if(xx == x && yy == x) {
      |            ^~
a.cc:24:18: error: 'x' was not declared in this scope
   24 |         if(xx == x && yy == x) {
      |                  ^
a.cc:24:23: error: 'yy' was not declared in this scope
   24 |         if(xx == x && yy == x) {
      |                       ^~
a.cc:25:17: error: 'cout' was not declared in this scope
   25 |                 cout<<-1;
      |                 ^~~~
a.cc:29:17: error: 'a' was not declared in this scope
   29 |                 a += yy;
      |                 ^
a.cc:30:17: error: 'b' was not declared in this scope
   30 |                 b += yy;
      |                 ^
a.cc:33:17: error: 'b' was not declared in this scope
   33 |                 b += x;
      |                 ^
a.cc:37:16: error: expected ';' before 'i'
   37 |         for (ll i = 39; i >= 0; i--){
      |                ^~
      |                ;
a.cc:37:25: error: 'i' was not declared in this scope
   37 |         for (ll i = 39; i >= 0; i--){
      |                         ^
a.cc:38:11: error: expected ';' before 'x'
   38 |         ll x = (1ll << i);
      |           ^~
      |           ;
a.cc:39:14: error: 'b' was not declared in this scope
   39 |         if(((b&x) == x) && ((a&x) == 0)){
      |              ^
a.cc:39:16: error: 'x' was not declared in this scope
   39 |         if(((b&x) == x) && ((a&x) == 0)){
      |                ^
a.cc:39:30: error: 'a' was not declared in this scope
   39 |         if(((b&x) == x) && ((a&x) == 0)){
      |                              ^
a.cc:40:23: error: 'A' was not declared in this scope
   40 |             if(b-x >= A[2]) b -= x, a += x;
      |                       ^
a.cc:44:8: error: 'a' was not declared in this scope
   44 |     if(a && b >= A[2])
      |        ^
a.cc:44:13: error: 'b' was not declared in this scope
   44 |     if(a && b >= A[2])
      |             ^
a.cc:44:18: error: 'A' was not declared in this scope
   44 |     if(a && b >= A[2])
      |                  ^
a.cc:45:9: error: 'cout' was not declared in this scope
   45 |         cout<<A[1] - a;
      |         ^~~~
a.cc:46:10: error: 'cout' was not declared in this scope
   46 |     else cout<<-1;newl;
      |          ^~~~
a.cc:46:19: error: 'newl' was not declared in this scope
   46 |     else cout<<-1;newl;
      |                   ^~~~