#include<bits/stdc++.h>
#define LL long long
#define int long long
#define orz cout << "tyy YYDS!\n"
using namespace std;
const int MAXN = 1e6 + 1000;
const int INF = 1e9 + 7;
const int mod = 998244353;

int n, y;
char s[MAXN];
int a[MAXN], b[MAXN], c[MAXN];
int f[MAXN];
int q[MAXN], head = 1, tail = 0;

int read() {
    int s = 0, f = 0;
    char ch = getchar();
    while(!isdigit(ch)) f |= (ch == '-'), ch = getchar();
    while(isdigit(ch)) s = (s << 1) + (s << 3) + (ch ^ 48), ch = getchar();
    return f ? -s : s;
}

int F(int x) { return f[x] - a[x] + b[x] * c[x]; }

double K(int x_1, int x_2) {
    if(b[x_1] == b[x_2]) return INF;
    return (1.0 * (F(x_1) - F(x_2))) / (1.0 * (b[x_1] - b[x_2]));
}

void Delete(int pos){ while(head < tail && 1.0 * c[pos] >= K(q[head], q[head + 1])) head ++; }

void Insert(int pos) {
    while(head < tail && K(q[tail - 1], q[tail]) > K(q[tail], pos)) tail --;
    q[++ tail] = pos;
}

signed main() {
    n = read(), y = read();
    cin >> s + 1;
//    s[++n] = '+', s[n + 1] = '\0';
    int x = 0;
    for(int i = 1; i <= n; ++i) {
        if(s[i] == '+') x ++;
        if(s[i] == '?') a[i] = x;
    }
    for(int i = 1; i <= n; ++i) a[i] += a[i - 1];
    for(int i = 1; i <= n; ++i) if(s[i] == '?') c[i] ++;
    for(int i = 1; i <= n; ++i) c[i] += c[i - 1];
    for(int i = 1; i <= n; ++i) if(s[i] == '+') b[i] ++;
    for(int i = 1; i <= n; ++i) b[i] += b[i - 1]; 
//    for(int i = 1; i <= n; ++i) cout << a[i] << " "; puts("");
//    for(int i = 1; i <= n; ++i) cout << b[i] << " "; puts("");
//    for(int i = 1; i <= n; ++i) cout << c[i] << " "; puts("");
    Insert(0);
    for(int i = 1; i <= n; ++i) {
//        if(s[i] == '+') continue;
        Delete(i);
        int j = q[head];
        f[i] = f[j] + a[i] - a[j] - (c[i] - c[j]) * b[j] + y;
//        cout << i << " " << j << " " << f[i] << "\n";
        Insert(i);
    }
    printf("%lld\n", f[n] - y);
    return 0;
}

/*

6 5
++??+?

6 8
++??+?

5 1
+++++

*/
//    for(int i = 1; i <= n; ++i) {
//        b[i] = b[i - 1] + (s[i] == '+');
//        if(s[i] == '?') a[i] = b[i];
//    }
//    for(int i = 1; i <= n; ++i) a[i] += a[i - 1];
//    for(int i = 1; i <= n; ++i) c[i] = c[i - 1] + (s[i] == '?');

//    for(int i = 1; i <= n; ++i) b[i] = s[i] == '+', c[i] = s[i] == '?';
//    for(int i = 1; i <= n; ++i) b[i] += b[i - 1], c[i] += c[i - 1];
//    for(int i = 1; i <= n; ++i) a[i] = a[i - 1] + b[i] * (s[i] == '?');