#include <iostream>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <queue>
#include <cstring>

template <typename Tp>
inline void read(Tp &x) {
    x = 0;
    bool f = true; char ch = getchar();
    for ( ; ch < '0' || ch > '9'; ch = getchar()) f ^= ch == '-';
    for ( ; ch >= '0' && ch <= '9'; ch = getchar()) x = x * 10 + (ch ^ 48);
    x = f ? x : -x;
}

const int N = 1e6;

long long sum[N + 5], dp[N + 5];
int a[N + 5], q[N + 5], id[N + 5];
char s[N + 5];

long long get(int i, int j) {
    return ((dp[i] - sum[i]) - (dp[j] - sum[j]));
}

int main() {
    int n, m;
    read(n), read(m);
    scanf("%s", s + 1);
    for (int i = 1; i <= n; ++i) {
        a[i] = a[i - 1];
        if (s[i] == '+') ++a[i];
    }
    for (int i = n; i >= 0; --i) {
        sum[i] = sum[i + 1];
        if (s[i] == '?') sum[i] += a[i];
    }
    int l = 1, r = 1, now = 0; q[l] = n + 1;
    for (int i = n; i >= 0; --i) {
        if (s[i] == '+') continue;
        id[i] = ++now;
        while (l < r && get(q[l + 1], q[l]) < 1LL * (-a[i]) * (id[q[l + 1]] - id[q[l]])) ++l;
        dp[i] = dp[q[l]] + sum[i] - sum[q[l]] - 1LL * a[i] * (id[i] - id[q[l]]);
        if (i) dp[i] += m;
        while (l < r && get(q[r], q[r - 1]) * (id[i] - id[q[r]]) > get(i, q[r]) * (id[q[r]] - id[q[r - 1]])) --r;
        q[++r] = i;
        // printf("%d %lld\n", q[l], dp[i]);
    }
    printf("%lld\n", dp[0]);
    return 0;
}