#include<bits/stdc++.h>
using namespace std;
#define ri int
typedef long long ll;
const int maxn=1e6+10;
template<class T>inline bool ckmin(T &x,const T &y){return x>y?x=y,1:0;}
template<class T>inline bool ckmax(T &x,const T &y){return x<y?x=y,1:0;}
template<class T>inline void clear(T *arr,int siz,int val=0){memset(arr,val,sizeof(T)*(siz+1));}
template<typename T>
struct dq{
	int hd,tl;
	T v[maxn];
	inline dq(){hd=1;}
	inline T front(int ex=0){return v[hd+ex];}
	inline T back(int ex=0){return v[tl-ex];}
	inline void pop_front(){++hd;}
	inline void pop_back(){--tl;}
	inline void push(T k){v[++tl]=k;}
	inline int size(){return tl-hd+1;}
	inline void clear(){hd=1,tl=0;}
};
dq<int>q;
int a[maxn],m,n,tot;
char s[maxn];
ll ans,f[maxn],suf[maxn],suf2[maxn];
inline ll K(int k){return n-k+1;}
inline ll X(int k){return suf[k+1];}
inline ll Y(int k){return f[k]+suf2[k+1];}
inline double slope(int u,int v){return 1.*(Y(v)-Y(u))/(X(v)-X(u));}
int main(){
	scanf("%d%d%s",&n,&m,s+1);
	for(ri i=1,j=0;i<=n;++i){
		if(s[i]=='?')a[++tot]=j,j=0;
		else ++j;
	}
	n=tot;
	for(ri i=n;i;--i){
		suf[i]=suf[i+1]+a[i];
		suf2[i]=suf2[i+1]+1ll*a[i]*(n-i+1);
	}
	ans=suf2[1];
	q.push(0);
	for(ri i=1;i<=n;++i){
		while(q.size()>1&&slope(q.front(),q.front(1))>K(i))q.pop_front();
		ri j=q.front();
		f[i]=f[j]+suf2[j+1]-suf2[i]-(n-i+1)*(suf[j+1]-suf[i])+m;
		ckmin(ans,f[i]+suf2[i+1]);
		while(q.size()>1&&slope(q.back(1),q.back())>slope(q.back(),i))q.pop_back();
		q.push(i);
	}
	printf("%lld",ans);
	return 0;
}