#include <bits/stdc++.h> using namespace std; int f[200][200]; int n,m,k,s,flag; int a[105],b[105]; 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<=m;u++) { f[v][u]=max(f[v][u],f[v-1][u-b[i]]+a[i]); } } } 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; } return 0; }