//用單調隊列優化
#include <iostream>
#define MAXV 10001
using namespace std;
typedef struct {
__int64 val;
int in;
}point;
point q[MAXV];
__int64 staf[MAXV];
__int64 cost[MAXV];
__int64 dp[MAXV], n;
__int64 c;
void res() {
int s = 0, e = -1;
dp[ 0 ] = 0;
point p, pt;
for(int i = 1; i <= n; ++ i) {
p.in = i;
p.val = cost[ i ];
while(s <= e) {
pt = q[ e ];
if(pt.val + c * (i - pt.in) < p.val) break;
e --;
}
++ e; q[ e ] = p; pt = q[ s ];
dp[ i ] = dp[i - 1] + staf[ i ] * (pt.val + c * (i - pt.in));
}
printf("%I64d\n", dp[ n ]);
}
int main() {
while(~scanf("%d %I64d", &n, &c)) {
for(int i = 1; i <= n; ++ i)
scanf("%I64d %I64d", &cost[ i ], &staf[ i ]);
res();
}
return 0;
}