PROG-1088 : กิ้งก่า (iguana)

#include <bits/stdc++.h>

using namespace std;

using pii = pair <int, int>;

int red, green, blue;
void cal(int val, int l){
    if(val % 3 == 1) red += l;
    else if(val % 3 == 2) green += l;
    else blue += l;
}

int main(){

    int n, m;
    scanf("%d%d", &n, &m);

    pii S[2*m + 1];

    for(int i=1;i<=m;i++){
        int a, b;
        scanf("%d%d", &a, &b);
        S[2*i - 1].first = a;
        S[2*i - 1].second = 1;
        S[2*i].first = b+1;
        S[2*i].second = 2;
    }

    char a;
    scanf(" %c", &a);

    sort(S + 1, S + 2*m + 1);

    int s = 1, val = 1;
    for(int i=1;i<=2*m;i++){
        int idx = S[i].first, opr = S[i].second;
        cal(val % 3, idx - s);
        s = idx;
        if(opr == 1) val ++;
        else val --;
    }
    cal(val, n + 1 - s);

    int sum;
    if(a == 'R') sum = (green * 2) + blue;
    else if(a == 'G') sum = red + (blue * 2);
    else sum = (red * 2) + green;
    printf("%d", sum);

    return 0;
}
/*
6 1
3 3
G

10 7
1 3
2 8
6 9
3 5
6 9
1 5
2 8
R

10 4
1 3
3 5
5 7
7 9
R

10 4
1 4
3 4
2 6
4 8
G

10 5
1 10
3 5
6 9
1 6
3 9
B

*/