#include<bits/stdc++.h>
using namespace std;
#define INF 0x3f3f3f3f
#define PI acos(-1)
#define io ios::sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define mem(a,b) memset((a),(b),sizeof(a))
typedef long long ll;
typedef unsigned long long ull;
#define int long long
#define db long double

const double eps = 1e-6;
const int mod = 1e9 + 7;
const int maxn = 1e5 + 10;

int n, y, bck[1001000];

string str;

void solve() {
    cin >> n >> y;
    cin >> str; str = "?" + str;
    for (int i = n; i >= 1; i--) {
        bck[i] = bck[i + 1] + (str[i] == '?');
    }
    int cnt = 0, ans = 0;
    for (int i = 1; i <= n; i++) {
        if (str[i] == '?') {
            if (cnt * bck[i] >= y) {
                cnt = 0;
                ans += y;
            }
            ans += cnt * bck[n];
        }
        if (str[i] == '+') {
            cnt++;
        }
    }
    cout << ans << '\n';
}

signed main(){
    io;
    solve();
    return 0;
}