#include <cstdio>
#include <algorithm>

#define MAX 100
int n, d, t[MAX], k[MAX], dp[MAX][MAX];

int main(){
    scanf("%d %d", &n, &d);

    for(int i = 0; i < n; i++){
        scanf("%d %d", &t[i], &k[i]);
    }

    dp[0][0] = t[0], dp[0][1] = k[0] - d;
    for(int i = 1; i < n; i++){
        dp[i][0] = std::max(dp[i - 1][0] + t[i], dp[i - 1][1] + t[i] - d);
        dp[i][1] = std::max(dp[i - 1][1] + k[i], dp[i - 1][0] + k[i] - d);
    }

    printf("%d\n", std::max(dp[n - 1][0], dp[n - 1][1]));

    return 0;
}