#include <stdio.h>
#include <stdbool.h>

int main(void){
	// 仕事日数・移動交通費
	int workday,expense;
	scanf("%d%d",&workday,&expense);
	//trueなら東京・falseなら京都
	bool locationflg = true;
	// 合計給料
	int total = 0;
	
	for(int i = 0;i < workday;i++){
		// 東京給料・京都給料
		int tokyoSalary,kyotoSalary;
		scanf("%d%d",&tokyoSalary,&kyotoSalary);	
		if(locationflg == true){
			if((kyotoSalary - expense) > tokyoSalary){
				total += (kyotoSalary - expense);
			}else{
				total += tokyoSalary;
			}
		}else{
			if((tokyoSalary - expense) > kyotoSalary){
				total += (tokyoSalary - expense);
			}else{
				total += kyotoSalary;
			}
		}
	}
	printf("%d",total);
}