#include <bits/stdc++.h>
using namespace std;

#ifdef CKISEKI
#define safe cerr << __PRETTY_FUNCTION__ << " line " << __LINE__ << " safe\n"
#define debug(a...) debug_(#a, a)
#define orange(a...) orange_(#a, a)
template <typename ...T>
void debug_(const char *s, T ...a) {
    cerr << "\e[1;32m(" << s << ") = (";
    int cnt = sizeof...(T);
    (..., (cerr << a << (--cnt ? ", " : ")\e[0m\n")));
}
template <typename I>
void orange_(const char *s, I L, I R) {
    cerr << "\e[1;32m[ " << s << " ] = [ ";
    for (int f = 0; L != R; ++L)
        cerr << (f++ ? ", " : "") << *L;
    cerr << " ]\e[0m\n";
}
#else
#define safe ((void)0)
#define debug(...) safe
#define orange(...) safe
#endif

const int maxn = 1000025;
int pre[maxn], suf[maxn];
int64_t dp[maxn];

int main() {
    cin.tie(nullptr)->sync_with_stdio(false);
    int n, y;
    cin >> n >> y;
    string s;
    cin >> s;

    for (int i = 0; i < n; i++) {
        pre[i + 1] = pre[i] + (s[i] == '+');
    }
    for (int i = n - 1; i >= 0; i--) {
        suf[i] = suf[i + 1] + (s[i] == '?');
    }

    dp[0] = 0;
    for (int i = 0; i < n; i++) {
        if (s[i] == '+')
            dp[0] += suf[i];
    }

    debug(dp[0]);

    int64_t ans = dp[0];

    deque<pair<int64_t,int64_t>> dq;

    const auto insert = [&dq](int64_t slope, int64_t b) {
        using pll = pair<int64_t,int64_t>;
        const auto ok = [](pll A, pll B, pll C) {
            assert (A.first <= B.first && B.first <= C.first);

            return (C.second - B.second) * (B.first - A.first) > (B.second - A.second) * (C.first - B.first);
        };

        pair<int64_t,int64_t> cur(slope, b);
        while (dq.size() >= 2 && !ok(dq[dq.size()-2], dq[dq.size()-1], cur))
            dq.pop_back();
        dq.emplace_back(cur);
    };

    const auto query = [&dq](int64_t x) {
        const auto f = [](pair<int64_t,int64_t> l, int64_t z) {
            return l.first * z + l.second;
        };
        while (dq.size() >= 2 && f(dq[0], x) >= f(dq[1], x))
            dq.pop_front();
        return f(dq[0], x);
    };

    insert(0, dp[0]);

    for (int i = 1; i <= n; i++) {
        dp[i] = 1e18;
        
        dp[i] = query(suf[i - 1]) + y - pre[i] * suf[i - 1];

        insert(pre[i], dp[i]);
        // for (int j = 0; j < i; j++) {
        //     dp[i] = min(dp[i], dp[j] - (pre[i] - pre[j]) * suf[i - 1] + y);
        // }


        debug(i, dp[i]);
        ans = min(ans, dp[i]);
    }

    cout << ans << '\n';

    return 0;
}