#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define X first
#define Y second
#define pb push_back
#define SZ(a) ((int)a.size())
#define ALL(v) v.begin(), v.end()

struct Line {
    ll a, b;
    ll operator()(ll x) const {
        return a * x + b;
    }
};

const int N = 1e6 + 5;
ll P[N], S[N], dp[N];

int main() {
    ios::sync_with_stdio(0), cin.tie(0);
    int n, y;
    cin >> n >> y;
    ll tt = 0;
    {
        string s;
        cin >> s;
        for (int i = 0, c = 0; i < n; i++)
            if (s[i] == '+')
                c++;
            else
                tt += c;
        P[0] = 0;
        for (int i = 1; i <= n; i++)
            P[i] = P[i - 1] + (s[i - 1] == '+');
        S[n + 1] = 0;
        for (int i = n; i >= 0; i--)
            S[i] = S[i + 1] + (s[i] == '?');
    }

   
    dp[0] = 0;
    deque<Line> dq;
    for (int i = 1; i <= n; i++) {
        Line l = {-P[i - 1], dp[i - 1]};
        while (true) {
            if (SZ(dq) >= 1 && dq.back().a == l.a) {
                if (l.b >= dq.back().b)
                    dq.pop_back();
                else
                    break;
            }
            else if (SZ(dq) >= 2 && (l.b - dq[SZ(dq) - 2].b) * (dq[SZ(dq) - 2].a - dq.back().a) > (dq.back().b - dq[SZ(dq) - 2].b) * (dq[SZ(dq) - 2].a - l.a))
                dq.pop_back();
            else {
                dq.push_back(l);
                break;
            }
        }
        assert(SZ(dq));
        while (SZ(dq) >= 2 && dq[0](S[i]) <= dq[1](S[i]))
            dq.pop_front();
        dp[i] = dq[0](S[i]) + (ll)S[i] * P[i] - y;
    }
    ll mx = 0;
    for (int i = 0; i <= n; i++)
        mx = max(mx, dp[i]);
    cout << tt - mx << endl;
}