#include <bits/stdc++.h>
typedef long long ll;
using namespace std;
int n,m,y,suf[1000010],a[1000010];
ll f[1000010],sufi[1000010];
char s[1000010];
int main(){
	scanf("%d%d%s",&n,&y,s+1);
	for(int i=1,j=0;i<=n;i++){
		if(s[i]=='?') a[++m]=j,j=0;
		else j++;
	}
	a[++m]=0;
	n=m;
	for(int i=n;i>=1;i--){
		suf[i]=suf[i+1]+a[i];
		sufi[i]=sufi[i+1]+a[i]*(n-i+1ll); 
	}
	f[0]=0;
	for(int i=1;i<=n;i++){
		f[i]=1e18;
		for(int j=1;j<=i;j++) f[i]=min(f[i],f[j-1]+sufi[j]+1ll*(i-1-n)*suf[j]-(i-1-n)*suf[i+1]-sufi[i+1]+y);
	}
	printf("%lld\n",f[n]-y);
}