#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int N=1000005;
int n;
ll y;
int a[N],tot=0;
ll f[N],ans=100000000000000ll;
int q[N],nowl=1,nowr=0;
struct Number{
	ll a,b;
};
Number Slope(int i,int j){
	ll x1=f[i]+(ll)a[i]*tot-f[j]-(ll)a[j]*tot;
	ll x2=a[i]-a[j];
	if(x2<0) x1=-x1,x2=-x2;
	return (Number){x1,x2};
}
bool Comp(Number k1,Number k2){
	return (k1.a*k2.b<k1.b*k2.a);
}
int main(){
	scanf("%d %lld\n",&n,&y);
	int lst=0;
	for(int i=1;i<=n;i++){
		char x;
		scanf("%c",&x);
		//cout<<i<<" "<<x<<endl;
		if(x=='?') a[++tot]=(i-lst-1),lst=i;
	}
	for(int i=1;i<=tot;i++) a[i]+=a[i-1];
	for(int i=1;i<=tot;i++) f[0]+=a[i];
	ans=min(ans,f[0]);//for(int i=1;i<=tot;i++) cout<<a[i]<<" ";cout<<endl;
	q[++nowr]=0;
	for(int i=1;i<=tot;i++){
		while(nowl<nowr && Comp(Slope(q[nowl],q[nowl+1]),(Number){i,1})) nowl++;
		int p=q[nowl];
		f[i]=f[p]+y-(ll)(a[i]-a[p])*(tot-i+1);ans=min(ans,f[i]);
		//cout<<i<<" zhuan: "<<p<<" "<<f[i]<<endl;
		while(nowl<nowr && Comp(Slope(q[nowr],i),Slope(q[nowr-1],q[nowr]))) nowr--;
		q[++nowr]=i;
	}
	cout<<ans<<endl;
	return 0;
}
/*
12 6
++++?+++?+??
*/