蒟蒻初学vector，为什么会出错？

#include<bits/stdc++.h>
using namespace std;
vector<int> a ;
unsigned long long s;
int main(){
	while(1){
		int x,y;
		cin>>x>>y;
		if(x==-1&&y==-1)return 0;
		for(int i=x;i<=y;i++){
			if(a[i]!=0){
				s+=a[i];
				continue;
			}
			int j=i,t=0;
			while(j){
				s+=j%10;
				t+=j%10;
				j/=10;
			}
			a[i]=t;
		}
		cout<<s<<endl;
		s=0;
	}
	return 0;
}