> #include <stdio.h>
> int a,b,c,d,e,f,g,h,i,j,k,l;
> int aa[10000],ab[10000],ac[10000];
> int main () {
>     while(1){
>     scanf("%d",&a);
>     if(a==-1)break;
>     for(i=0;i<a;i++){
>                      scanf("%d%d",&aa[i],&ab[i]);
>                      }
>     for(i=a-1;i>0;i--){
>                        ab[i]=ab[i]-ab[i-1];
>                        }
>     for(i=0;i<a;i++){
>                      b+=aa[i]*ab[i];
>                      }
>     ac[c]=b;
>     c++;
>     b=0;
> }
>     for(i=0;i<c;i++){
>                      printf("%d miles\n",ac[i]);
>                      }
> }