#include<iostream>
#include<cstdio>
#include<vector>
#include<cstring>
using namespace std;
bool vis[1000];int go(int maxdis,vector< vector<int> >& stations)
{int mx=0,pos=-1,num=stations.size(),i=0;while(i<num&&stations[i][0]<=maxdis){if(stations[i][1]>mx&&!vis[i]){mx=stations[i][1];pos=i;}i++;}if(pos==-1)return -1;else{vis[pos]=1;return mx;}}
int minRefuelStops(int target, int startFuel, vector< vector<int> >& stations) {memset(vis,0,sizeof(vis));//把终点当作一个加油站vector<int>v;v.push_back(target);v.push_back(0);stations.push_back(v);int num=stations.size();//cout<<num<<endl;if(startFuel>=target)return 0;else{//beg是当前加油的下标，i是可以走到的加油站下标int dis=startFuel,i=0,start=0,cou=0,beg=0;while(start+dis<target){//cout<<"start: "<<start<<"  "<<i<<endl;//计算每次行驶完所有的汽油后可以经过的加油站，在含油量最高的加油站处加油//cout<<stations[i][0]<<endl;int mx=0,po=-1;int maxdis=start+dis;  //可以走的最远距离//找出第一个到达不了的加油站while(i<num&&stations[i][0]<=maxdis){i++;}//不断加油直到可以走到while(i<num&&start+dis<stations[i][0]){int tmp=go(maxdis, stations);if(tmp!=-1){dis+=tmp;cou++;}elsebreak;}if(start+dis>=stations[i][0]){dis-=(stations[i][0]-start);start=stations[i][0];beg=i;}elsebreak;}if(start+dis>=target)return cou;elsereturn -1;}
}int main()
{vector< vector<int> > stations;int target,startFuel;while(1){scanf("%d%d",&target,&startFuel);vector<int>s;s.push_back(14);s.push_back(123);stations.push_back(s);s.clear();s.push_back(145);s.push_back(203);stations.push_back(s);s.clear();s.push_back(344);s.push_back(26);stations.push_back(s);s.clear();s.push_back(357);s.push_back(68);stations.push_back(s);s.clear();s.push_back(390);s.push_back(35);stations.push_back(s);s.clear();s.push_back(478);s.push_back(135);stations.push_back(s);s.clear();s.push_back(685);s.push_back(108);stations.push_back(s);s.clear();s.push_back(823);s.push_back(186);stations.push_back(s);s.clear();s.push_back(934);s.push_back(217);stations.push_back(s);s.clear();s.push_back(959);s.push_back(80);stations.push_back(s);printf("%d\n",minRefuelStops(target,startFuel,stations));}
}