#include <stdio.h>
#include <stdbool.h>
#include <stdlib.h>

#define MAX_SEGMENTS 100 /* m is guaranteed to be between 1 and 100 */
#define MAX_HEIGHT 1000

bool isHeigherWallFromLeft(const int wall[], const int length,
    const int currentBlock, const int currentHeight)
{
    for (int i = 0; i < currentBlock; i++)
    {
        if (wall[i] >= currentHeight)
        {
            return true;
        }
    }

    return false;
}

bool isHeigherWallFromRight(const int wall[], const int length,
    const int currentBlock, const int currentHeight)
{
    for (int i = currentBlock + 1; i < length; i++)
    {
        if (wall[i] >= currentHeight)
        {
            return true;
        }
    }

    return false;
}

int findNumberOfWaterBlocks(const int wall[], const int length)
{
    int numberOfWaterBlocks = 0;

    for (int i = 1; i < length - 1; i++)
    {
        for (int j = wall[i] + 1; j < MAX_HEIGHT; j++)
        {
            if (isHeigherWallFromRight(wall, length, i, j) &&
                isHeigherWallFromLeft(wall, length, i, j))
            {
                numberOfWaterBlocks++;
            }
            else
            {
                break;
            }
        }
    }

    return numberOfWaterBlocks;
}

int buildWall(int wall[], const int length)
{
    int numberOfFilled = 0, i, height;
    while (scanf("%d %d", &i, &height) == 2)
    {
        if (height > 0 && i >= 0 && i < length && height <= MAX_HEIGHT)
        {
            wall[i] += height;
            numberOfFilled++;
        }
    }

    return numberOfFilled;
}

int calc_trapped_water()
{
    int m, wall[MAX_SEGMENTS] = {0};

    scanf("%d", &m);
    if (buildWall(wall, m))
    {
        return findNumberOfWaterBlocks(wall, m);
    }

    return 0;
}

int main()
{
    printf("%d\n", calc_trapped_water());
    return 0;
}