简单粗暴，直接上代码

#include<iostream>
#include <string>
#include <unordered_map>
using namespace std;
int main() {
    string str;
    char ch;
    unordered_map<char,int> unmap;

    getline(cin,str);
    cin>>ch;
    if (ch >= 'A' && ch <= 'Z') {
        ch = ch - 'A' + 'a';
    }

    for (int i=0;i<str.size();++i) {
        if (str[i] >= 'A' && str[i] <= 'Z')
            str[i] = str[i] -'A' + 'a';
        unmap[str[i]]++;
    }
    cout<<unmap[ch]<<endl;
    return 0;
}