2019 CCPC 秦皇岛 MUV LUV EXTRA kmp

#include<iostream>
#include<algorithm>
#include<string.h>
#include<string>
using namespace std;
const int N = 1e7 + 5;
typedef long long ll;
char s[N];
char s1[N];
int to[N];
void get_to() {
	to[1] = 0;
	int k = 0, n = strlen(s + 1);
	for (int i = 2; i <= n; i++) {
		while (k && s[i] != s[k + 1]) k = to[k];
		if (s[i] == s[k + 1]) k++;
		to[i] = k;
	}
}
int main() {
	ll a, b;

	while (~scanf("%lld%lld", &a, &b)) {

		scanf("%s", s1 + 1);
		int n = strlen(s1 + 1);
		int tot = 0;
		for (int i = n; i > 0; i--) {
			if (s1[i] == '.') break;
			s[++tot] = s1[i];
		}
		s[++tot] = '\0';
		//printf("%s\n", s + 1);
		get_to();
		n = strlen(s + 1);
		ll cnt = -1e18;
		for (int i = n; i; i--) {
			ll tmp = to[i];
			cnt = max(cnt, a * i - b * (i - tmp));
		}
		cout << cnt << endl;
	}
	return 0;
}