#include<bits/stdc++.h> using namespace std; const int N=200; int f[N][N],n,m,k,s,a[N],b[N]; int main(){ while(cin>>n>>m>>k>>s){ memset (f,0,sizeof f); for(int i=1;i<=k;i++)cin>>a[i]>>b[i]; for(int i=1;i<=k;i++){ for(int v=1;v<=s;v++){ for(int u=b[i];u<=n;u++)f[v][u]=max(f[v][u],f[v-1][u-b[i]]+a[i]); } } int flag=-1; for(int u=0;u<=m;u++){ for(int v=1;v<=s;v++){ if(f[v][u]>=n) { flag=m-u; u=m+1; break; } } } cout<<flag<<endl; } }