#include<iostream>
#include<algorithm>
#include<cstring>
#include<cstdio>
#include<bitset>
#include<cmath>
#include<ctime>
#include<queue>
#include<map>
#include<set>

#define int long long
#define fi first
#define se second
#define max Max
#define min Min
#define abs Abs
#define mid ((l+r)>>1)
#define pb(x) push_back(x)
#define lowbit(x) (x&(-x))
#define fan(x) (((x-1)^1)+1)
#define mp(x,y) make_pair(x,y)
#define clr(f,n) memset(f,0,sizeof(int)*(n))
#define cpy(f,g,n) memcpy(f,g,sizeof(int)*(n))
#define INF 0x3f3f3f3f

using namespace std;

inline int read()
{
	int ans=0,f=1;
	char c=getchar();
	while(c>'9'||c<'0'){if(c=='-')f=-1;c=getchar();}
	while(c>='0'&&c<='9'){ans=(ans<<1)+(ans<<3)+c-'0';c=getchar();}
	return ans*f;
}

inline void write(int x)
{
	if(x<0) putchar('-'),x=-x;
	if(x/10) write(x/10);
	putchar((char)(x%10)+'0');
}

template<typename T>inline T Abs(T a){return a>0?a:-a;};
template<typename T,typename TT>inline T Min(T a,TT b){return a<b?a:b;}
template<typename T,typename TT> inline T Max(T a,TT b){return a<b?b:a;}

const int N=1e6+5;
int n,m,k,a[N],b[N],dp[N],all,ans,rt;
char s[N];

struct Line
{
	int k,b;
}p[N];

inline int f(int x,int y)
{
	return p[x].k*y+p[x].b;
}

struct Seg
{
	int val[N*30],lc[N*30],rc[N*30],cnt;
	inline void update(int &x,int l,int r,int v)
	{
		if(!x) x=++cnt;
		if(l==r)
		{
			if(f(v,l)<f(val[x],l))
				val[x]=v;
			return;
		}
		if(p[v].k<=p[val[x]].k)
		{
			if(f(v,mid)<f(val[x],mid))
				update(lc[x],l,mid,val[x]),val[x]=v;
			else update(rc[x],mid+1,r,v);
		}
		else
		{
			if(f(v,mid)<f(val[x],mid))
				update(rc[x],mid+1,r,val[x]),val[x]=v;
			else update(lc[x],l,mid,v);
		}
	}
	inline int query(int x,int l,int r,int p)
	{
		if(!x) return 1e18;
		int res=f(val[x],p);
		if(l==r) return res;
		if(p<=mid) res=min(res,query(lc[x],l,mid,p));
		else res=min(res,query(rc[x],mid+1,r,p));
		return res;
	}
}sum;

signed main()
{
	n=read();k=read();
	scanf("%s",s+1);
	for(int i=1;i<=n;++i)
		a[i]=a[i-1]+(s[i]=='+');
	for(int i=n;i>=1;--i)
		b[i]=b[i+1]+(s[i]=='?');
	for(int i=1;i<=n;++i)
		if(s[i]=='?') all+=a[i];
	for(int i=1;i<=n;++i)
	{
		dp[i]=a[i]*b[i]-k;
		dp[i]=max(dp[i],a[i]*b[i]-k-sum.query(rt,0,n,b[i]));
		p[++m]=(Line){a[i],-dp[i]};sum.update(rt,0,n,m);
		ans=max(ans,dp[i]);
	}
	write(all-ans);
	return 0;
}