use strict;
use warnings;

my $num = <>;
my @fare = ();
my $count = 0;
my $pas = 0;
my $ans = 0;
while(<>){
    if(++$count < $num){
        push @fare, $_;
    }else{
        / /;
        $pas -= $`;
        $pas += $';
        $ans += $pas * shift @fare;
    }
}
print $ans."\n";