#include<bits/stdc++.h>
using namespace std;
constexpr int M=1e6+5;
int n,m,ans=INT_MAX,h[M],stk[M];
int cal(int x,int y,int d){
    x=x?(x-1)/d+1:0; y=y?(y-1)/d+1:0;
    int ans=0; while (x||y){ ans++;
        if (x&1||!y) x>>=1,(++y)>>=1;
        else (++x)>>=1,y>>=1;
    }
    return ans;
}
int calc(int xl,int xr,int yl,int yr){
    return cal(xl,n-xr-1,xr-xl+1)+cal(yl,m-yr-1,yr-yl+1);
}
int main(){
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    cin>>n>>m;stk[0]=-1;
    for (int i=0;i<n;i++){
        string str;cin>>str;
        for (int j=0;j<m;j++)
            h[j]=str[j]=='1'?h[j]+1:0;
        for (int j=0,tp=0;j<=m;j++){
            while (tp&&h[stk[tp]]>h[j])
                ans=min(ans,calc(i-h[stk[tp]]+1,i,stk[tp-1]+1,j-1)),tp--;
            stk[++tp]=j;
        }
    }
    printf("%d\n",ans);
    return 0;
}