#include <bits/stdc++.h>
typedef long long ll;
typedef __int128 lll; 
using namespace std;
int n,m,y,suf[1000010],a[1000010],l,r,q[1000010];
ll f[1000010],sufi[1000010],X[1000010],Y[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;
	l=1,r=0;
	for(int i=1;i<=n;i++){
		f[i]=1e18;
		Y[i]=f[i-1]+sufi[i]-(1ll+n)*suf[i],X[i]=-suf[i];
		while(l<r&&(X[q[r-1]]-X[i])*(Y[q[r]]-Y[i])-(Y[q[r-1]]-Y[i])*(X[q[r]]-X[i])<=0) r--;
		q[++r]=i;
		while(l<r&&(Y[q[l+1]]-Y[q[l]])<=i*(X[q[l+1]]-X[q[l]])) l++;
		f[i]=Y[q[l]]-i*X[q[l]];
		f[i]+=y-(i-1ll-n)*suf[i+1]-sufi[i+1];
	}
	printf("%lld\n",f[n]-y);
}