#include<bits/stdc++.h>
#define ALL(v) std::begin(v),std::end(v)
using lint=long long;
using ld=long double;
int main(){
    std::cin.tie(nullptr);std::ios_base::sync_with_stdio(false);
    std::cout.setf(std::ios_base::fixed);std::cout.precision(15);
    lint n,k;std::cin>>n>>k;
    std::vector<lint>a(2*n),cost(2*n+1),stock(2*n+1);
    for(lint i=0;i<n;i++){
        char c;std::cin>>c;
        a.at(i)=a.at(i+n)=c-'0';
    }
    for(lint i=0;i<2*n;i++){
        lint s=stock.at(i);
        lint c=cost.at(i);
        if(s)s--;
        else c++;
        s+=a.at(i);
        stock.at(i+1)=s;
        cost.at(i+1)=c;
    }
    if(auto found=std::find(stock.begin()+n,stock.end(),0);found==a.end()){
        std::cout<<cost.at(std::min(k,2*n))<<'\n';
    }else{
        lint q=std::max(0ll,k/n-1);
        std::cout<<(cost.at(2*n)-cost.at(n))*q+cost.at(k-n*q)<<'\n';
    }
}