将第五行代码改为long long就会过掉,为什么ull不行呢? 代码如下

#include<iostream>
using namespace std;
int main(){
	char X,Z;
	unsigned long long Y, W;
	cin>>X>>Y>>Z>>W; 
	if(X == Z && (X >= 'a' && X <= 'z')){
		cout<<"valid"<<endl;
		if(Y <= W)cout<< W-Y+1<<endl;
		else {
			cout<< Y-W+1<<endl;
		}
	}
	else{
		cout<<"Invalid"<<endl<<-1<<endl;
	}
	return 0;
}