#include<bits/stdc++.h>
using namespace std;
char s[1000005];
int S[1000005],T[1000005];
long long f[1000005];
int q[1000005];
double slope(int x,int y){
	return 1.0*(f[y]+1ll*S[y]*T[y]-f[x]-1ll*S[x]*T[x])/(T[y]-T[x]);
}
int main(){
	int n,y;
	cin>>n>>y;
	scanf("%s",s+1);
	for(int i=1;i<=n;++i){
		S[i]=S[i-1]+(s[i]=='?');
		T[i]=T[i-1]+(s[i]=='+');
	}
	int head=0,tail=-1;
	q[++tail]=0;
	for(int i=1;i<=n;++i){
		while(head<tail&&slope(q[head],q[head+1])<S[i])++head;
		int zy=q[head];
		f[i]=f[zy]+1ll*(S[i]-S[zy])*T[zy]-y;
		f[i]=max(f[i],0ll);
		while(head<tail&&slope(q[tail-1],q[tail])>slope(q[tail],i))--tail;
		q[++tail]=i;
	}
	long long ans=0;
	for(int i=1;i<=n;++i){
		if(s[i]=='?')ans+=T[i-1];
	}
	cout<<ans-f[n]<<endl;
	return 0;
}