C++ :

#include <iostream>
using namespace std;
int main() {
    int k, m, n, s, answer = 0;
    cin >> m >> n;
    for (int i = m; i <= n; i ++) {
        s = 0;
        k = i;
        while (k) {
            s = s + k % 10;
            k = k / 10;
        }
        if (s == 7) answer++;
    }
    cout << answer;
    return 0;
}

Python :

# coding=utf-8
m = int(input())
n = int(input())
s = 0
f=0
for i in range(m,n+1):
    s =0
    a = len(str(i))
    for j in range(1,a+1):
        s=s+i//10**(j-1)%10
    if s==7 :
        f+=1
print(f)