#include<bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a),i##z=(b);i<=i##z;i++)
#define ROF(i,a,b) for(int i=(a),i##z=(b);i>=i##z;i--)
#define REP(i,u) for(int i=hd[u],v;v=to[i],i;i=nxt[i])
#define temT template<typename T>
#define temT12 template<typename T1,typename T2>
using namespace std;
typedef pair<int,int> pii;
typedef long long ll;
typedef long double ld;
const int N=int(1e6)+10;
const ll linf=ll(1e17)+10;

ll f0[N],pa[N],pq[N],f[N],g[N]; char a[N]; int n,y;

ll calc(ll j,ll i){
	return f0[i]-f0[j]+y+f[j]+pa[j]*pq[j]-pa[j]*pq[i];
}

struct node{ int l,r,i; }s[N]; int hd,tl;

int check(int x,int i){
	if(calc(i,s[x].r)>=calc(s[x].i,s[x].r))
		return s[x].r+1;
	int l=s[x].l,r=s[x].r,mid,res;
	while(l<=r){
		mid=(l+r)/2;
		if(calc(i,mid)<calc(s[x].i,mid))
			res=mid,r=mid-1;
		else
			l=mid+1;
	}
	return res;
}

int main(){
	cin>>n>>y; FOR(i,1,n) while(a[i]!='+' && a[i]!='?') cin>>a[i];
	FOR(i,1,n) pa[i]=pa[i-1]+(a[i]=='+');
	FOR(i,1,n) pq[i]=pq[i-1]+(a[i]=='?');
	FOR(i,1,n) f0[i]=f0[i-1]+(a[i]=='?'?pa[i]:0);
	/*
	FOR(i,1,n){
		f[i]=f0[i];
		FOR(j,0,i-1) f[i]=min(f[i],f0[i]-f0[j]+y+f[j]+pa[j]*pq[j]-pa[j]*pq[i]);
	}
	*/
	
	FOR(i,1,n) f[i]=f0[i]; f[0]=-y;
	hd=tl=1,s[1]=(node){1,n,0};
	FOR(i,1,n){
		f[i]=min(f[i],calc(s[hd].i,i));
		s[hd].l++; if(s[hd].l>s[hd].r) hd++;
		int pos=s[hd].l;
		ROF(x,tl,hd)
			if(calc(i,s[x].l)>=calc(s[x].i,s[x].l)){ pos=check(x,i); break; }
		if(pos<=n){
			while(s[tl].l>=pos && tl>=hd) tl--;
			if(hd<=tl) s[tl].r=pos-1;
			s[++tl]=(node){pos,n,i};
		}
	}
	
	cout<<f[n];
	return 0;
}