// tested by Hightail - https://github.com/dj3500/hightail
import std.stdio, std.string, std.conv, std.algorithm;
import std.range, std.array, std.math, std.typecons, std.container, core.bitop;

immutable inf = 10^^9 + 7;

void main() {
    int n, d;
    long[] t, k;

    scan(n, d);
    t = new long[](n);
    k = new long[](n);

    foreach (i ; 0 .. n) {
        scan(t[i], k[i]);
    }

    auto dp = new long[][](n + 1, 2);
    dp[0][1] = -inf;

    foreach (i ; 0 .. n) {
        foreach (j ; 0 .. 2) {
            if (j == 0) {
                dp[i + 1][j] = max(dp[i + 1][j], dp[i][j] + t[i]);
                dp[i + 1][j ^ 1] = max(dp[i + 1][j ^ 1], dp[i][j] - d + k[i]);
            }
            else {
                dp[i + 1][j] = max(dp[i + 1][j], dp[i][j] + k[i]);
                dp[i + 1][j ^ 1] = max(dp[i + 1][j ^ 1], dp[i][j] - d + t[i]);
            }
        }
    }

    writeln(max(dp[n][0], dp[n][1]));
}



void scan(T...)(ref T args) {
    string[] line = readln.split;
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}


void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}