#include<cstdio>
#include<iostream>
using namespace std;
const int o=1e6+10;
int n,y,a[o],q[o],Q[o],hd,tl;long long ans,f[o],g[o];char s[o];
inline long long calc(int t,int qi){return qi*1ll*a[t]-g[t];}
int main(){
	scanf("%d%d%s",&n,&y,s+1);
	for(int i=1;i<=n;++i) a[i]=a[i-1]+(s[i]=='+'),q[i]=q[i-1]+(s[i]=='?');
	for(int i=1;i<=n;Q[++tl]=i++){
		if(s[i]=='?') ans+=a[i];
		for(;hd<tl&&calc(Q[hd],q[i])<calc(Q[hd+1],q[i]);++hd);
		if(hd<=tl) f[i]=max(0ll,calc(Q[hd],q[i])-y);
		g[i]=a[i]*1ll*q[i]-f[i];
		for(;hd<tl&&(g[i]-g[Q[tl]])*(a[Q[tl]]-a[Q[tl-1]])<=(g[Q[tl]]-g[Q[tl-1]])*(a[i]-a[Q[tl]]);--tl);
	}
	printf("%lld",ans-f[n]);
	return 0;
}