#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;
	// 東京の日給
	int tokyoSalary[workday];
	// 京都の日給
	int kyotoSalary[workday];
	
	for(int i = 0;i < workday;i++){
		scanf("%d%d",&tokyoSalary[i],&kyotoSalary[i]);
	}
	
	for(int i = 0;i < workday;i++){	
		if(locationflg == true){
			if((kyotoSalary[i] - expense) > tokyoSalary[i]){
				total += (kyotoSalary[i] - expense);
			}else if((kyotoSalary[i] - expense) < tokyoSalary[i]){
				total += tokyoSalary[i];
			}else if((kyotoSalary[i] - expense) == tokyoSalary[i]){
				if(kyotoSalary[i] > tokyoSalary[i]){
					total += (kyotoSalary[i] - expense);
				}else{
					total += tokyoSalary[i];
				}
			}
		}else{
			if((tokyoSalary[i] - expense) > kyotoSalary[i]){
				total += (tokyoSalary[i] - expense);
			}else if((tokyoSalary[i] - expense) < kyotoSalary[i]){
				total += kyotoSalary[i];
			}else if((tokyoSalary[i] - expense) == kyotoSalary[i]){
				if(tokyoSalary[i] > kyotoSalary[i]){
					total += (tokyoSalary[i] - expense);
				}else{
					total += kyotoSalary[i];
				}
			}
		}
	}
	printf("%d",total);
}