// #pragma GCC optimize("O3")
#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pb push_back
#define N 1000000007
#define N2 998244353
#define Nmax 100005
#define INF (ll)1e18
#define pll pair<ll,ll>
#define mp make_pair
#define fi first
#define se second
#define vv vector
#define rep(i,n) rep2(i,0,n)
#define rep2(i,m,n) for(ll i=m;i<(n);i++)
#define repr(i,n) repr2(i,n,0)
#define repr2(i,m,n) for(ll i=m;i>=(n);i--)
#define get(n) ll (n); scanf("%lld",&(n));
#define get2(n,k) ll (n),(k); scanf("%lld%lld",&(n),&(k));
#define get3(n,m,k) ll (n),(m),(k); scanf("%lld%lld%lld",&(n),&(m),&(k));
#define gets(s) string (s); cin >> (s);
#define ALL(c) (c).begin(),(c).end()
#define fill_nums(A,n) vector<ll> A((n)); for(ll i=0;i<n;i++) scanf("%lld",&A[i]);
#define fill_edges(adj,m) for(ll i=0;i<m;i++) {ll a, b; scanf("%lld %lld",&a,&b); adj[a-1].pb(b-1); adj[b-1].pb(a-1);}
#define fill_direct(adj,m) for(ll i=0;i<m;i++) {ll a, b; scanf("%lld %lld",&a,&b); adj[a-1].pb(b-1);}
#define StarBurstStream ios_base::sync_with_stdio(false); cin.tie(0); //cout.tie(0);
#define pi 3.1415926535897L
#define db long double  /* print with %Lf */
#define EPS 1e-9
#define flu fflush(stdout)
#define imp {cout<<"No\n"; return 0;}
#define yup {cout<<"Yes\n"; return 0;}
#define debug_vec(vec,n) {rep(idxi,n) cout<<vec[idxi]<<" "; cout<<"\n";}
#define debug_pvec(vec,n) {rep(idxi,n) cout<<vec[idxi].fi<<"-"<<vec[idxi].se<<"  "; cout<<"\n";}
#define debug_set(seta) {cout<<seta.size()<<"\n"; for(auto idxi:seta) cout<<idxi<<" "; cout<<"\n";}
#define debug_map(mappa) {cout<<mappa.size()<<"\n";for(auto idxi:mappa) cout<<idxi.fi<<"-"<<idxi.se<<"  "; cout<<"\n";}
#define debug_vecvec(vec,n) {rep(idxi,n) {for(auto idxi2:vec[idxi]) cout<<idxi2<<" "; cout<<"\n";}}
 
// LOOK AT THESE BEFORE SUBMISSION
// LOOK AT THESE BEFORE SUBMISSION
// ============================================================
// Nmax value and is index access offset taking it beyond Nmax.
// Precision error in case of real numbers.
// N vs N2 chosen correctly.
// Modulo applied everywhere.
// Flush in case of interactive.
// Don't erase value in multiset.
// Check for array indices out of bound (in pre or suff sums).
// Correct return value for functions.
// Confirm the letter case of yes and no.
// Dont paste code two times in the text file.
// ============================================================

vector<vector<ll>> dp(15, vector<ll>(1005,0));
vector<vector<ll>> vals(15, vector<ll>(1005,0));

int main() {

    get3(h,w,k);
    vector<string> A(h);
    rep(i,h) cin >> A[i];  

    rep(i,h) {
        rep(j,w) {
            vals[i][j] = (i-1>=0 ? vals[i-1][j] : 0) + (A[i][j]=='1');
        }
    }

    ll mini = INF;
    rep(i,1<<(h-1)) {
        vector<ll> cuts(1,0);
        rep(j,h-1) {
            if(i & (1<<j)) cuts.pb(j+1);
        }
        cuts.pb(h);
        // debug_vec(cuts, cuts.size());

        ll d = cuts.size()-1;
        rep(j, d) {
            rep(k, w) {
                ll v1 = cuts[j], v2 = cuts[j+1];
                dp[j][k] = vals[v2-1][k] - (v1-1>=0 ? vals[v1-1][k] : 0);
            }
        }

        ll ans = 0;
        vector<ll> sum(d,0);
        bool ff = 0;
        rep(t,w) {
            bool f = 0;

            rep(j,d) {
                if(dp[j][t]>k) {
                    ff=1; break;
                }

                sum[j] += dp[j][t];
                if(sum[j] > k) f = 1;
            }
            if(ff) break;

            if(f) {
                rep(j,d) {
                    sum[j] = dp[j][t];             
                } 
                ans++;
            }

        }
        if(ff) continue;

        mini = min(mini, ans + d - 1);
    }   

    printf("%lld\n", mini);
    return 0;  
}   


Measured memory_bytes: 512