C :

#include<stdio.h>
int n,m,mid,a[100001],r[100001];
void sort(int s,int t)
{
	if(s==t) return;
	int m=(s+t)/2;
	sort(s,m);
	sort(m+1,t);
	int i=s;
	int j=m+1;
	int k=s;
	while(i<=m&&j<=t)
	{
		if(a[i]<=a[j]) r[k++]=a[i++];
	    else r[k++]=a[j++];
	}
	while(i<=m) r[k++]=a[i++];
	while(j<=t) r[k++]=a[j++];
	for(i=s;i<=t;i++) a[i]=r[i];
}
int judge()
{
	int t=a[1]-m;
	if(t<0) t=0;
	for(int i=2;i<=n;i++)
	{
		t+=mid;
		if(a[i]<t) return 0;
	}
	return 1;
}
main()
{
	scanf("%d%d",&n,&m);
	for(int i=1;i<=n;i++) scanf("%d",&a[i]);
	sort(1,n);
	int l=0,r=10000000;
	if(r>a[n]) r=a[n];
	mid=(l+r)>>1;
	while(l<r-1)
	{
		if(judge()) l=mid;
		else r=mid;
		mid=(l+r)>>1;
	}
	printf("%d",mid);
}

C++ :

#include <cstdio>
#include <algorithm>

#define  maxm  10000000
#define  maxn  100000

int n , m ;
int L = 1 , R , M ;
int a[maxn] ;

int main() {
	//#define READ
	#ifdef  READ
		freopen("sniper.in" , "r" , stdin ) ;
		freopen("sniper.out", "w" , stdout) ;
	#endif
	scanf("%d%d", &n , &m ) ;
	for (int i = 1 ; i <= n ; i ++ ) scanf("%d", &a[i] ) ;
	std::sort(a+1,a+n+1) ; R = a[n] ;
	int pos , flag ;
	while (L != R) {
		M = (L+R)>>1 ;
		if (M == L) M++ ;
		//printf("M = %d\n",M);
		pos = -M ; flag = false ;
		for (int i = 1 ; i <= n ; i ++ ) {
			//printf("%d\n",a[i]);
			pos += M ;
			if (pos > a[i]) {
				R = M-1 ; flag = true ; break ;
			}
			if (a[i]-m-pos > 0) pos += a[i]-m-pos ;
		}
		if (flag) continue ;
		L = M ;
	}
	printf("%d\n", L ) ;
	return 0 ;
}
/*
6 100
236
120
120
120
120
120
*/