问个让新手头疼的问题

#include<iostream>
#include<string>
using namespace std;
void main(){
    string s;
    char S;
    cout<<"输入字符串："<<endl;
    cin>>S;
    while(S!='$'){
        s=s+S;
        cin>>S;
    }
    cout<<s<<endl;
}
输入$号应该退出循环呀，为什么还能输入，要打回车才退出呢