为什么会爆零啊？

#include<bits/stdc++.h>
using namespace std;
int b[26]={1,2,3,1,2,3,1,2,3,1,2,3,1,2,3,1,2,3,4,1,2,3,1,2,3,4};
int main()
{
	string a;
	getline(cin,a);
	int sum=0;
	for(int i=0;i<a.size();i++)
	{
		if(a[i]>='a'&&a[i]<='z')
			sum+=b[a[i]-'a'];
		else
			sum++;
	}
	cout<<sum<<endl;
}
//i see dead people
//31