#include <stdio.h>
int main(void){
    int days, koutsu;
    scanf( "%d %d", &days, &koutsu );

    int t[100];
    int k[100];
    int i = 0;
    int hikaku[2];
    int hikaku_prev[2];

    for( i = 0; i < days; i++ ){
        int input_t, input_k;
        scanf( "%d %d", &input_t, &input_k );
        t[i] = input_t;
        k[i] = input_k;
    }
    
    // 初期処理
    hikaku_prev[0] = t[0];
    hikaku_prev[1] = k[0] - koutsu;
    hikaku[0] = 0;
    hikaku[1] = 0;
 
    for( i = 1; i < days; i++ ){
        //東京の仕事で期待できる最大の値を得る
        // 移動しない場合と移動した場合
        if( hikaku_prev[0] > hikaku_prev[1] - koutsu ){
            hikaku[0] = t[i] + hikaku_prev[0];
        }else{
            hikaku[0] = t[i] + hikaku_prev[1] - koutsu;
        }
        //京都の仕事で期待できる最大の値を得る
        // 移動しない場合と移動した場合
        if( hikaku_prev[1] > hikaku_prev[0] - koutsu ){
            hikaku[1] = k[i] + hikaku_prev[1];
        }else{
            hikaku[1] = k[i] + hikaku_prev[0] - koutsu;
        }
        //保存
    	hikaku_prev[0] = hikaku[0];
    	hikaku_prev[1] = hikaku[1];
    }

    if( hikaku_prev[0] >= hikaku_prev[1] ){
        printf( "%d\n", hikaku_prev[0] );
    }else{
        printf( "%d\n", hikaku_prev[1] );
    }
    return 0;
}