#include<stdio.h>struct{ __int64 x,y;}a[100];__int64 map[100][100];int main(){ int w,s,p,i,j,k; while(scanf("%d",&w)!=EOF,w) { for(i=0;i<=88;i++) { a[i].x=2000000001; for(j=0;j<=88;j++) map[i][j]=2000000001; } scanf("%d%d",&s,&p); j=p; for(i=1;i<=p;i++) { j++; scanf("%I64d%I64d",&a[i].x,&a[i].y); map[i][j]=0; } a[0].x=a[0].y=0; a[2*p+1].x=a[2*p+1].y=w; __int64 sum,count; for(i=0;i<=0;i++) { sum=0; count=0; for(j=i+1;j<=p+1;j++) { if(j==p+1) { if(w>sum) { if((w-sum)%s!=0) count++; count=count+(w-sum)/s; } map[i][2*p+1]=count; break; } if(a[j].x>sum) { int tt=count; if((a[j].x-sum)%s!=0) count++; count=count+(a[j].x-sum)/s; sum+=(count-tt)*s; } if(a[j].x==sum) { map[i][j]=count; sum--; } else if(a[j].x<sum) { map[i][j]=count; while(a[j+1].x<sum) { j++; map[i][j]=count; } if(a[j+1].x==sum) { j++; map[i][j]=count; int jj=j,kk=s; sum--; while(a[jj-1].x==sum) { kk--; map[i][jj]=count; sum--;jj--; if(kk==1) break; } if(kk==1) break; } } } } for(i=1;i<=p;i++){ j=1; sum=a[i].y; count=0; while(a[i].y>a[j].x||i==j) j++; for(j;j<=p+1;j++) { if(j==p+1) { if(w>sum) { if((w-sum)%s!=0) count++; count=count+(w-sum)/s; } map[i+p][2*p+1]=count; break; } if(sum<a[j].x) { int tt=count; if((a[j].x-sum)%s!=0) count++; count=count+(a[j].x-sum)/s; sum+=(count-tt)*s; } if(sum==a[j].x) { map[i+p][j]=count; sum--; } else if(sum>a[j].x) { map[i+p][j]=count; while(a[j+1].x<sum) { j++; map[i+p][j]=count; } if(a[j+1].x==sum) { j++; map[i+p][j]=count; int jj=j,kk=s; sum--; while(a[jj-1].x==sum) { kk--; map[i+p][jj]=count; sum--;jj--; if(kk==1) break; } if(kk==1) break; } } } } for(i=0;i<=2*p+1;i++) { for(j=0;j<=2*p+1;j++) { for(k=0;k<=2*p+1;k++) { if(map[j][k]>map[j][i]+map[i][k]) map[j][k]=map[j][i]+map[i][k]; } } } printf("%I64dn",map[0][2*p+1]);} return 0;}
