为什么差分WA？

#include <iostream>
#include <cstdio>
#include <cstdlib>
using namespace std;
#define mid (l+r)/2
int len,n,m,a[50001],ans;
bool ok(int x)
{
    int k=0,b[50001]; for (int i=1;i<=n;i++) b[i]=a[i];
    for (int i=1;i<=n;i++) {if (b[i]<x) {k++; b[i+1]+=b[i];}}
    return k<=m;
}
int main()
{
    cin>>len>>n>>m; for (int i=1;i<=n;i++) cin>>a[i];
    a[++n]=len; for (int i=n;i>=1;i--) a[i]-=a[i-1];
    int l=0,r=len;
    while(l<=r) {if (ok(mid)) {ans=mid; l=mid+1;} else r=mid-1;}
    cout<<ans<<endl;
return 0;
}