#include <array> // array, swap
#include <bit> // (C++20) popcount
#include <cstdlib> // abort
#include <iostream> // cin, cout, endl
#include <string> // string

#define MOD 998244353
#define KMAX 26

using ull = unsigned long long;

int main() {
	ull n, m, k;
	std::string s;
	std::cin >> n >> m >> k >> s;
	
    if(n < 1 || n > m || m > 500000 || s.length() != n || k < 1 || k > KMAX) {
        std::abort();
    }

	ull kbits = 0, result = 0;
	std::array<ull, KMAX> dp, ndp;
	dp.fill(0);

	for(auto it = s.begin(); it != s.end(); it++) {
		ull ci = *it - 'a';
		if(ci >= KMAX) {
		    std::abort;
		}
		ndp.fill(0);
		for(ull j = 0; j < ci; j++) {
			ndp[std::popcount(kbits | (1 << j)) - 1] += 1;
		}
		kbits |= (1 << ci);
		if(std::popcount(kbits) >= k) {
			result += 1;
		}
		for(ull j = 1; j < KMAX; j++) {
			ndp[j - 1] += j * dp[j - 1];
			ndp[j] += (KMAX - j) * dp[j - 1];
		}
		ndp[KMAX - 1] += KMAX * dp[KMAX - 1];
		for(ull j = 0; j < KMAX; j++) {
			ndp[j] %= MOD;
		}
		for(ull j = k - 1; j < KMAX; j++) {
			result += ndp[j];
		}
		result %= MOD;
		std::swap(dp, ndp);
	}

	if(std::popcount(kbits) >= k) {
		result = (result + (MOD - 1)) % MOD;
	}

	for(ull i = n; i < m; i++) {
		ndp.fill(0);
		for(ull j = 1; j < KMAX; j++) {
			ndp[j - 1] += j * dp[j - 1];
			ndp[j] += (KMAX - j) * dp[j - 1];
		}
		ndp[KMAX - 1] += KMAX * dp[KMAX - 1];
		for(ull j = 0; j < KMAX; j++) {
			ndp[j] %= MOD;
		}
		for(ull j = k - 1; j < KMAX; j++) {
			result += ndp[j];
		}
		result %= MOD;
		std::swap(dp, ndp);
	}
	
	std::cout << result << std::endl;
}