CF621C (求概率)

#include <iostream>#include <cstring>#include <cstdio>#include <algorithm>using namespace std;double f[123456];int main(){    int n,p;    int beg,ed;    scanf("%d%d",&n,&p);    for(int i=0;i<n;i++){        scanf("%d%d",&beg,&ed);        int l=ed-beg+1;        f[i]=((ed)/p-(beg-1)/p)*1.0/l;    }    double w=0;    for(int i=0;i<n-1;i++){        w+=(f[i]*f[i+1]+(1-f[i])*f[i+1]+f[i]*(1-f[i+1]))*2000;    }    w+=(f[0]*f[n-1]+(1-f[0])*f[n-1]+f[0]*(1-f[n-1]))*2000;    printf("%f",w);    return 0;}