#include <bits/stdc++.h> using namespace std; int f[200][200]; int main() { //freopen("game.in","r",stdin); //freopen("game.out","w",stdout); int n,m,k,s; int a[105],b[105]; while(~scanf("%d%d%d%d",&n,&m,&k,&s)) { memset(f,0,sizeof(f)); for(int i=1; i<=k; i++) scanf("%d%d",&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]); 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; } printf("%d\n",flag); } return 0; }