#include<bits/stdc++.h>
#define ll long long
#define all(a) (a).begin(),(a).end()
using namespace std;

#define int ll

#define x(j) (j - cnt[j])
#define y(j) (f[j] - pre[j] + j * cnt[j] - cnt[j] * cnt[j])
const int N = 1e6 + 10;
int n, y, pre[N], cnt[N], f[N];
int head, que[N], p;

void solve()
{
    cin >> n >> y;
    string s;
    cin >> s;
    s = '?' + s;

    for(int i = 1; i <= n; i++)
    {
        pre[i] = pre[i - 1];
        cnt[i] = cnt[i - 1];
        if(s[i] == '?')
        {
            cnt[i]++;
            pre[i] += i - cnt[i];
        }
    }

    int ans = pre[n];

    p = 1;
    que[++head] = 0;
    for(int i = 1; i <= n; i++)
    {
        if(s[i] == '+') continue;
        int k = cnt[i - 1];

        while(p < head && k * (x(que[p + 1])) - x(que[p]) >= y(que[p + 1]) - y(que[p])) p++;

        int j = que[p];
        f[i] = y(j) - x(j) * k + y + pre[i - 1];

        while(p < head && (y(que[head]) - y(que[head - 1])) * (x(i) - x(que[head])) >= (y(i) - y(que[head])) * (x(que[head] - x(que[head - 1])))) head--;
        que[++head] = i;

        ans = min(ans, f[i] + pre[n] - pre[i] - (i - cnt[i]) * (cnt[n] - cnt[i]));
        // cerr << pre[n] - pre[i] - (i - 1) * (cnt[n] - cnt[i]) << '\n';

        // cerr << f[i] << ' ' << ans << '\n';
    }

    cout << ans << '\n';
}

signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);cout.tie(nullptr);
    int t = 1;
    //cin >> t;
    while(t--) solve();
    return 0;
}