package backjoon;
import java.util.Scanner;
public class Apple {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int schoolCount = scanner.nextInt();
int result = 0;
for (int i = 0; i < schoolCount; i++) {
int studentsCount = scanner.nextInt();
int appleCount = scanner.nextInt();
result += appleCount % studentsCount;
}
System.out.println(result);
}
}