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

typedef struct {
    int pos;
    int height;
} Restriction;

int compare(const void* a, const void* b) {
    return ((Restriction*)a)->pos - ((Restriction*)b)->pos;
}

int min(int a, int b) { return a < b ? a : b; }
int max(int a, int b) { return a > b ? a : b; }

int maxBuilding(int n, Restriction* restrictions, int size) {
    Restriction* all = malloc((size + 2) * sizeof(Restriction));
    
    // Copy existing restrictions
    for (int i = 0; i < size; i++) {
        all[i] = restrictions[i];
    }
    
    // Add boundaries
    all[size] = (Restriction){1, 0};
    all[size + 1] = (Restriction){n + 1, n - 1};
    size += 2;
    
    qsort(all, size, sizeof(Restriction), compare);
    
    // Forward pass
    for (int i = 1; i < size; i++) {
        int maxFromLeft = all[i-1].height + (all[i].pos - all[i-1].pos);
        all[i].height = min(all[i].height, maxFromLeft);
    }
    
    // Backward pass
    for (int i = size - 2; i >= 0; i--) {
        int maxFromRight = all[i+1].height + (all[i+1].pos - all[i].pos);
        all[i].height = min(all[i].height, maxFromRight);
    }
    
    // Find maximum
    int maxHeight = 0;
    for (int i = 0; i < size - 1; i++) {
        int distance = all[i+1].pos - all[i].pos;
        int peakHeight = (all[i].height + all[i+1].height + distance) / 2;
        maxHeight = max(maxHeight, peakHeight);
    }
    
    free(all);
    return maxHeight;
}

int main() {
    int n;
    scanf("%d", &n);
    Restriction restrictions[1000];
    printf("%d\n", maxBuilding(n, restrictions, 0));
    return 0;
}