#include<iostream>
#include<cstdio>
#include<algorithm>
#include<string.h>
#include<queue>
#include<vector>
#include<map>
#include<bitset>
#include<set>
#include<cmath>
#include<ctime>
#include<random>
#define vi vector<int>
#define pb push_back
#define fi first
#define se second
#define mp make_pair
#define bc(x) __builtin_popcount(x)
#define re register
#define il inline
#define pii pair<int,int>
#define pil pair<int,long long>
#define pll pair<long long,long long>
#define mem0(x) memset(x,0,sizeof(x))
#define mem0x3f(x) memset(x,0x3f,sizeof(x))
#define dbg(x) cerr<<"In Line "<< __LINE__<<" the "<<#x<<" = "<<x<<'\n';
// #pragma GCC optimize(3)
#define int long long
typedef long long ll;
typedef unsigned long long ull;
using namespace std;
namespace IO_BUFF{
	mt19937 rnd(time(0)^(ll)(new char));
	int rend(int x){
		return rnd()%x+1;
	}
	void rendom_shuffle(int *a,int len){
		shuffle(a+1,a+len+1,rnd);
	}
	const int BS=(1<<24)+5;char Buffer[BS],*HD,*TL;
	inline int gc(){
	    if(HD==TL) TL=(HD=Buffer)+fread(Buffer,1,BS,stdin);
	    return (HD==TL)?EOF:*HD++;
	}
	inline int inn(){
	    int x,ch,s=1;while((ch=gc())<'0'||ch>'9')if(ch=='-')s=-1;x=ch^'0';
	    while((ch=gc())>='0'&&ch<='9') x=(x<<1)+(x<<3)+(ch^'0');return x*s;
	}
	char ssss[19999999],tttt[20];int ssl,ttl;
    inline int print(int x)
    {
        if(x<0)ssss[++ssl]='-',x=(-x);
		if(!x) ssss[++ssl]='0';for(ttl=0;x;x/=10) tttt[++ttl]=char(x%10+'0');
        for(;ttl;ttl--) ssss[++ssl]=tttt[ttl];return ssss[++ssl]='\n';
    }
	inline int Flush(){return fwrite(ssss+1,sizeof(char),ssl,stdout),ssl=0,0;}
	int read(){
		char c=getchar();int x=1;int s=0;
		while(c<'0' || c>'9'){if(c=='-')x=-1;c=getchar();}
		while(c>='0' && c<='9'){
			s=s*10+c-'0';c=getchar();
		}
		return s*x;
	}
}using namespace IO_BUFF;
/*namespace CFConTest{
	const int mod=998244353;
	inline int add(const int &x,const int &y){
		return (x+y>=mod?x+y-mod:x+y);
	}
	inline int del(const int &x,const int &y){
		return (x-y<0?x-y+mod:x-y);
	}
	int ksm(int x,int k){
		int base=1;
		while(k){
			if(k&1)base=1ll*base*x%mod;
			k>>=1;
			x=1ll*x*x%mod;
		}
		return base;
	}
};
using namespace CFConTest;*/
const int N=1e6+5;
int n,m,f[N];
char a[N];
int s[N],c[N],d[N],e[N],g[N];
int st[N];
double solve(int x,int y){
	return ((f[x]+g[x])-(f[y]+g[y]));
}
double solp(int x,int y){
	if(d[x]==d[y]){
		if((f[x]+g[x])>=(f[y]+g[y]))return 1e15;
		return -1e15;
	}
	return 1.0*((f[x]+g[x])-(f[y]+g[y]))/(d[x]-d[y]);
}
signed main(){
	#ifdef newbiewzs
		freopen("data.in","r",stdin);
	#else
	#endif
	n=read();m=read();
	scanf("%s",a+1);
	for(int i=1;i<=n;i++){
		c[i]=c[i-1];
		d[i]=d[i-1];
		s[i]=s[i-1];
		if(a[i]=='+')d[i]++;
		if(a[i]=='?')c[i]++;
		if(a[i]=='?')s[i]+=d[i];
		e[i]=c[i]*d[i];
		g[i]=e[i]-s[i];
	}
	memset(f,10,sizeof(f));
	f[0]=0;
	int head=1,tail=0;
	st[++tail]=0;
	int tou=1;
	for(int i=1;i<=n;i++){
		while(head<tail && 1.0*c[i]*(d[st[head+1]]-d[st[head]])>solve(st[head+1],st[head]))head++;
		int k=st[head];
		f[i]=s[i]+m-c[i]*d[k]+f[k]+g[k];
		while(head<tail && solp(i,st[tail])<solp(st[tail],st[tail-1]))tail--;
		st[++tail]=i;
	}
	int ans=1e15;
	for(int i=0;i<n;i++){
		ans=min(ans,f[i]+(s[n]-s[i])-(c[n]-c[i])*d[i]);
	}
	cout<<ans;
	return 0;
}