#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>

int main()
{
	int t = 0,res=0;
	scanf("%d", &t);
	for (int i = 0; i < t; i++)
	{
		int stu = 0, apple = 0;
		scanf("%d %d", &stu, &apple);
		res += apple % stu;
	}
	printf("%d", res);
	return 0;
}