#include <bits/stdc++.h>

using namespace std;

typedef pair<int, int> pii;
typedef long long ll;
const int N = 2000086, MOD = 998244353, INF = 0x3f3f3f3f;
ll res;
int n, m, cnt, w[N];

struct ST {
    struct Node {
        int l, r;
        ll sum;
    };
    vector<Node> tr;

    ST(ST&& other) noexcept : tr(move(other.tr)) { }
    ST (int l, int r) : tr(vector<Node>(((r - l) << 2) + 10)) { build(1, l, r); }

    ST& operator=(ST&& other) noexcept {
        if (this != &other) {
            tr = move(other.tr);
        }
        return *this;
    }

private:
    void pushup(int u) { tr[u].sum = tr[u << 1].sum + tr[u << 1 | 1].sum; }

    void build(int u, int l, int r) {
        tr[u] = {l, r};
        if (l == r) return;
        int mid = l + r >> 1;
        build(u << 1, l, mid), build(u << 1 | 1, mid + 1, r);
        pushup(u);
    }

public:
    ll query(int u, int l, int r) {
        if (tr[u].l >= l && tr[u].r <= r) return tr[u].sum;
        int mid = tr[u].l + tr[u].r >> 1;
        ll res = 0;
        if (l <= mid) res += query(u << 1, l, r);
        if (r > mid) res += query(u << 1 | 1, l, r);
        return res;
    }

    void modify(int u, int x, int v) {
        if (tr[u].l == tr[u].r) tr[u].sum += v;
        else {
            int mid = tr[u].l + tr[u].r >> 1;
            if (x <= mid) modify(u << 1, x, v);
            else modify(u << 1 | 1, x, v);
            pushup(u);
        }
    }
};

char s[N];
ll k;
int f[N];
int main() {
    ST tr(-400000, 1200);
    cin >> n >> k;
    scanf("%s", s + 1);
    int c0 = 0;
    for (int i = 1; i < n + 1; i++) {
        if (s[i] == '0') c0++;
        else {
            int l = -200000, r = 1200;
            while (l < r) {
                int mid = l + r + 1 >> 1;
                if (mid - c0 + tr.query(1, mid, 1200) < 1200) l = mid;
                else r = mid - 1;
            }
            f[i] = l;
            tr.modify(1, l, 1);
        }
    }
    int now = 1200;
    ll t = 0;
    vector<pii> v = {{1200, 0}};
    set<int> st = {1200};
    while (1) {
        int c = tr.query(1, now, 1200);
        if (t + c >= k) {
            for (int i = 1; i < n + 1; i++) {
                t += f[i] >= now;
                if (t == k) {
                    printf("%lld\n", res + i);
                    return 0;
                }
            }
            assert(0);
        }

        int ne = now + c - c0;
        if (st.count(ne)) {
            for (int i = 0; i < v.size(); i++)
                if (v[i].first == ne) {
                    ll g = 0, len = 0;
                    v.push_back({ne, c});
                    for (int j = i + 1; j < v.size(); j++) g += v[j].second, len++;
                    t += c, res += n;
                    res += (k - t) / g * n * len;
                    t += (k - t) / g * g;
                    if (t == k) res -= n * len, t -= g;

                    for (int j = i + 1; j < v.size(); j++) {
                        if (t + v[j].second < k) t += v[j].second;
                        else {
                            for (int l = 1; l < n + 1; l++) {
                                t += f[l] >= v[j - 1].first;
                                if (t == k) {
                                    printf("%lld\n", res + l);
                                    return 0;
                                }
                            }
                        }
                    }
                    assert(0);
                }
            assert(0);
        } else if (c == n - c0 && ne < now) {
            ll g = (k - t) % c;
            if (!g) g += c;
            res += (k - t) / c * n;
            t += (k - t) / c * c;
            if (t == k) res -= n, t -= c;

            for (int i = 1; i < n + 1; i++) {
                t += s[i] == '1';
                if (t == k) {
                    printf("%lld\n", res + i);
                    return 0;
                }
            }
            assert(0);
        } else {
            v.push_back({ne, c});
            st.insert(ne);
            t += c;
            now = ne;
        }
        res += n;
    }
    return 0;
}