题解

#include <iostream>
#include <string>
#include <vector>

void solve() {
    int k;
    std::cin >> k;
    std::string s;
    char a1, a2;
    std::cin >> s >> a1 >> a2;

    int n = s.length();
    if (n < k) {
        std::cout << 0 << '\n';
        return;
    }

    std::vector<int> prefix_a1(n, 0);

    if (s[0] == a1) {
        prefix_a1[0] = 1;
    }

    for (int i = 1; i < n; ++i) {
        prefix_a1[i] = prefix_a1[i - 1] + (s[i] == a1 ? 1 : 0);
    }

    long long total_count = 0;
    for (int j = k - 1; j < n; ++j) {
        if (s[j] == a2) {
            int start_limit_index = j - k + 1;
            if (start_limit_index >= 0) {
                total_count += prefix_a1[start_limit_index];
            }
        }
    }

    std::cout << total_count << '\n';
}

int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(NULL);
    solve();
    return 0;
}