#pragma GCC optimize("Ofast","unroll-loops","omit-frame-pointer","inline")
#pragma GCC option("arch=native","tune=native","no-zero-upper")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2")
#pragma GCC optimize(3)
#include<bits/stdc++.h>
#define N 1000010
#define LL long long
#define rep(i,l,r) for(int i=l;i<=r;i++)
#define drep(i,r,l) for(int i=r;i>=l;i--)
using namespace std;
int rd() {
	int res=0,f=1;char ch=getchar();
	while(ch<'0'||ch>'9') {if(ch=='-') f*=-1;ch=getchar();}
	while(ch>='0'&&ch<='9') res=(res<<1)+(res<<3)+(ch^48),ch=getchar();
	return res*f;
}
int n,y;
char s[N];
int cnt1[N],cnt2[N];LL sum[N];
LL dp[N],X[N],Y[N];
int q[N],hd=1,tl=0;
int main() {
	n=rd(),y=rd();
	scanf("%s",s+1);
	rep(i,1,n) {
		cnt1[i]=cnt1[i-1]+(s[i]=='?');
		cnt2[i]=cnt2[i-1]+(s[i]=='+');
		sum[i]=sum[i-1]+(s[i]=='?')*cnt2[i];
	}
	
	q[++tl]=0,X[0]=0,Y[0]=0;
	rep(i,1,n) {
		LL K=cnt1[i];
		while(hd<tl&&K*(X[q[hd+1]]-X[q[hd]])>=(Y[q[hd+1]]-Y[q[hd]])) hd++;
		int j=q[hd];
		dp[i]=dp[j]+sum[i]-sum[j]-1ll*(cnt1[i]-cnt1[j])*cnt2[j]+y;
		X[i]=cnt2[i],Y[i]=dp[i]-sum[i]+1ll*cnt1[i]*cnt2[i];
		while(hd<tl&&(Y[q[tl]]-Y[q[tl-1]])*(X[i]-X[q[tl-1]])>=(Y[i]-Y[q[tl-1]])*(X[q[tl]]-X[q[tl-1]])) tl--;
		q[++tl]=i;
	}
	printf("%lld",dp[n]-y);
	return 0;
}