#include <bits/stdc++.h>

using namespace std;
using LL = long long;

const int N = 1e6+5;
constexpr LL INF = 0x3f3f3f3f3f3f3f3f;

int n;

namespace LiChaoSegmentTree
{
	struct Line {
		LL k, b;
		LL calc(LL x) const { return k*x+b; }
	}t[4*N];

	#define lc (x<<1)
	#define rc (x<<1|1)

	void update(int x, int l, int r, Line qval)
	{
		int mid = (l+r)/2;
		// fprintf(stderr, "* (%d, %d)\n", l, r);
		if(qval.calc(l) < t[x].calc(l) && qval.calc(r) < t[x].calc(r)) {
			t[x] = qval;
		} else if(qval.calc(l) < t[x].calc(l)) {
			if(qval.calc(mid) > t[x].calc(mid)) 
				update(lc, l, mid, qval);
			else swap(t[x], qval), update(rc, mid+1, r, qval); 
		} else if(qval.calc(r) < t[x].calc(r)) {
			if(qval.calc(mid+1) > t[x].calc(mid+1)) 
				update(rc, mid+1, r, qval);
			else swap(t[x], qval), update(lc, l, mid, qval);
		} 
	}

	LL query(int x, int l, int r, int qpos)
	{
		if(l == r) return t[x].calc(qpos);
		LL res = t[x].calc(qpos);
		int mid = (l+r)/2;
		if(qpos <= mid) res = min(res, query(lc, l, mid, qpos));
		else res = min(res, query(rc, mid+1, r, qpos));
		return res;
	}

	void addSegment(Line qval) { update(1, 0, n, qval); }
	LL queryPoint(int qpos) { return query(1, 0, n, qpos); }
	void build() 
	{
		for(int i = 1; i < 4*N; i++) 
			t[i].k = 0, t[i].b = INF;
	}
}
using LiChaoSegmentTree::Line;

LL m;
char str[N];
LL cnta[N], cntq[N], sum[N];
LL f[N];

int main()
{
	// freopen("1.in","r",stdin);
	scanf("%d%lld", &n, &m);
	scanf(" %s", str+1);

	for(int i = 1; i <= n; i++) {
		cnta[i] = cnta[i-1]+(str[i] == '+');
		cntq[i] = cntq[i-1]+(str[i] == '?');
		sum[i] = sum[i-1]+(str[i] == '?')*cnta[i];
	}

	f[0] = -m;
	q[++tt] = 0;

	// f[i] = f[j]+(sum[i]-sum[j])-(cntq[i]-cntq[j])*cnta[j]+m;
	// f[i]-sum[i]-m = -cnta[j] * cntq[i] + cnta[j]*cntq[j]-sum[j]+f[j];
	LiChaoSegmentTree::build();
	auto getLine = [&](int j) { 
		return Line{-cnta[j], cnta[j]*cntq[j]-sum[j]+f[j]}; 
	};
	LiChaoSegmentTree::addSegment(getLine(0));
	for(int i = 1; i <= n; i++) {
		f[i] = LiChaoSegmentTree::queryPoint(cntq[i])+sum[i]+m;
		LiChaoSegmentTree::addSegment(getLine(i));
	}
	printf("%lld\n", f[n]);
	return 0;
	return 0;
}

answer.code: In function ‘int main()’:
answer.code:76:9: error: ‘q’ was not declared in this scope
   76 |         q[++tt] = 0;
      |         ^
answer.code:76:13: error: ‘tt’ was not declared in this scope; did you mean ‘tm’?
   76 |         q[++tt] = 0;
      |             ^~
      |             tm
answer.code:66:14: warning: ignoring return value of ‘int scanf(const char*, ...)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
   66 |         scanf("%d%lld", &n, &m);
      |         ~~~~~^~~~~~~~~~~~~~~~~~
answer.code:67:14: warning: ignoring return value of ‘int scanf(const char*, ...)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
   67 |         scanf(" %s", str+1);
      |         ~~~~~^~~~~~~~~~~~~~