#include <bits/stdc++.h>

typedef long long ll;
typedef unsigned long long ull;
#define FOR(i, a, b) for (int i = a; i <= b; ++i)
#define ROF(i, a, b) for (int i = a; i >= b; --i)

inline int read() {
    int f = 1, x = 0;
    char ch = getchar();
    while (ch < '0' || ch > '9') {
        if (ch == '-') f = -1;
        ch = getchar();
    }
    while (ch >= '0' && ch <= '9') {
        x = (x << 1) + (x << 3) + (ch ^ 48);
        ch = getchar();
    }
    return x * f;
}

const int N = 1e6 + 7;
using namespace std;
int n;
ll f[N], g[N], W;
char ch[N];
ll w[N], s[N], c[N];
int q[N], hd, tl;
ll X(int i) { return w[i]; }
ll Y(int i) { return g[i]; }
double slope(int i, int j) { return X(i) == X(j) ? 1e18 : (Y(i) - Y(j)) / (X(i) - X(j)); }
int main() {
    n = read(), W = read();
    scanf("%s", ch + 1);
    FOR(i, 1, n) {
        w[i] = w[i - 1] + (ch[i] == '+');
        c[i] = c[i - 1] + (ch[i] == '?');
    }
    FOR(i, 1, n) { s[i] = s[i - 1] + w[i] * (ch[i] == '?'); }
    memset(f, 0x3f, sizeof(f));
    f[0] = 0;
    // FOR(i, 1, n) {
    //     int id = 0;
    //     FOR(j, 0, i - 1) {
    //         if (g[j] - c[i] * w[j] + s[i] + W < f[i]) {
    //             f[i] = g[j] - c[i] * w[j] + s[i] + W;
    //             id = j;
    //         }
    //     }
    //     printf("%d %d %lld\n", i, id, f[i]);
    //     g[i] = f[i] - s[i] + c[i] * w[i];
    // }
    q[hd = tl = 1] = 0;
    FOR(i, 1, n) {
        while (hd < tl && slope(q[hd], q[hd + 1]) < c[i]) ++hd;
        f[i] = g[q[hd]] - c[i] * w[q[hd]] + s[i] + W;
        g[i] = f[i] - s[i] + c[i] * w[i];
        while (hd < tl && slope(q[tl - 1], i) < slope(q[tl - 1], q[tl])) --tl;
        q[++tl] = i;
    }
    printf("%lld\n", f[n] - W);
    return 0;
}