#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#define ll long long
#define N 4000005
using namespace std;
ll res[N],cnt[N],sum[N],dp[N];
int n;
ll y;
ll pty[N],ptx[N];
int q[N],fr,ed;
// double slope(int i,int r){

// }
char s[N];
ll final_ans=0;
int main(){
  scanf("%d%lld",&n,&y);
  scanf("%s",s+1);
  for(int i=1;i<=n;i++)dp[i]=1e18;
  for(int i=1;i<=n;i++){
    sum[i]=sum[i-1];
    cnt[i]=cnt[i-1];
    res[i]=res[i-1];
    if(s[i]=='?'){
      sum[i]++;
      res[i]+=cnt[i];
    }
    else cnt[i]++;
  }
  final_ans=res[n];
  fr=1,ed=0;
  q[++ed]=1;
  ptx[1]=pty[1]=0;dp[1]=0;
  for(int i=2;i<=n;i++){
    // if(s[i]=='+')continue;
    while(fr<ed&&pty[q[fr+1]]-pty[q[fr]]<=sum[i-1]*(ptx[q[fr+1]]-ptx[q[fr]])){
      dp[i]=min(dp[i],dp[q[fr]]+y+res[i-1]-res[q[fr]-1]-(sum[i-1]-sum[q[fr]-1])*cnt[q[fr]-1]);
      fr++;
    }
    dp[i]=min(dp[i],dp[q[fr]]+y+res[i-1]-res[q[fr]-1]-(sum[i-1]-sum[q[fr]-1])*cnt[q[fr]-1]);
    ptx[i]=cnt[i-1];
    pty[i]=dp[i]-res[i-1]+sum[i-1]*cnt[i-1];
    final_ans=min(final_ans,dp[i]+res[n]-res[i-1]-(sum[n]-sum[i-1])*cnt[i-1]);
    while(fr<ed&&(pty[q[ed]]-pty[q[ed-1]])*(ptx[i]-ptx[q[ed-1]])>=(pty[i]-pty[q[ed-1]])*(ptx[q[ed]]-ptx[q[ed-1]]))ed--;
    q[++ed]=i;
  }
  printf("%lld\n",final_ans);
}